Send media audio to the car without a per-drive prompt

The head unit offers a media sink, but Android answers Setup with an
empty reply instead of a Config message. AudioLink now falls back to
the advertised sink after 2.5s. AudioCaptureService captures by audio
usage, not by app, so switching media apps costs nothing further and
one grant covers a whole drive.

A one-time adb command (PROJECT_MEDIA) removes the consent dialog
itself; the setup screen now includes it as a copyable step, next to
the existing mic-permission step.

Capture counts as a screen share, so Android 15 hides notification
contents while it runs. ScreenShareGuard lifts that protection for
the length of the capture, with a second adb grant
(WRITE_SECURE_SETTINGS), and restores it afterward, including after a
crash.

The phone keeps playing what it captures, so AudioCaptureService also
mutes STREAM_MUSIC for the drive and restores it on every exit path.

Adds a debug bench (BenchAudioSink, CarPreviewActivity --ez audio
true) for testing the audio path without a car, and a unit test that
rebuilds the real head unit's ServiceDiscoveryResponse from a logged
session to confirm the media and guidance sinks are found correctly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

gearslip/src/debug/java/app/seb3thehacker/gearslip/BenchAudioSink.kt

@@ -0,0 +1,62 @@
+package app.seb3thehacker.gearslip
+
+import app.seb3thehacker.gearslip.audio.CarAudio
+import kotlin.math.abs
+
+/**
+ * Debug-only stand-in for the head unit's media audio sink.
+ *
+ * The car end of the audio path can only be exercised in a car, but everything before it - the
+ * setting, the consent dialog, the capture service, and whether the media app allows itself to be
+ * heard at all - is phone-side and can be tested at a desk. This sink takes the place of
+ * [AudioLink] so that path runs to completion, and reports what arrived.
+ *
+ *   adb shell am start -n app.seb3thehacker.gearslip/.CarPreviewActivity \
+ *       --ei w 800 --ei h 480 --ez audio true --ei snaps 10
+ *
+ * A healthy run logs a rising byte count and a peak well above zero. A byte count that rises with
+ * a peak of zero means the app refused to be captured.
+ */
+class BenchAudioSink(override val sampleRate: Int = 48_000, override val channels: Int = 2) : CarAudio.Link {
+
+    private var chunks = 0
+    private var bytes = 0L
+    private var peak = 0
+    private var reportedAt = 0L
+
+    override fun start() {
+        chunks = 0
+        bytes = 0
+        peak = 0
+        reportedAt = System.currentTimeMillis()
+        GearslipLog.i("bench sink: started - pretending to be a ${sampleRate}Hz x$channels car sink")
+    }
+
+    override fun stop() {
+        GearslipLog.i("bench sink: stopped after $chunks chunks / $bytes bytes")
+    }
+
+    override fun write(pcm: ByteArray, size: Int) {
+        chunks++
+        bytes += size
+        for (i in 0 until size - 1 step 2) {
+            val sample = (pcm[i].toInt() and 0xFF) or (pcm[i + 1].toInt() shl 8)
+            peak = maxOf(peak, abs(sample.toShort().toInt()))
+        }
+        val now = System.currentTimeMillis()
+        if (now - reportedAt < REPORT_MS) return
+        reportedAt = now
+        val seconds = bytes / (sampleRate.toDouble() * channels * 2)
+        GearslipLog.i(
+            "bench sink: %d chunks / %d bytes (%.1fs of audio), peak %.3f%s".format(
+                chunks, bytes, seconds, peak / 32768f,
+                if (peak == 0) " - SILENT, the app is not letting itself be captured" else "",
+            ),
+        )
+        peak = 0
+    }
+
+    companion object {
+        private const val REPORT_MS = 2_000L
+    }
+}

gearslip/src/debug/java/app/seb3thehacker/gearslip/CarPreviewActivity.kt

@@ -9,6 +9,9 @@ import android.os.Handler
 import android.os.Looper
 import android.view.MotionEvent
 import androidx.activity.ComponentActivity
+import androidx.activity.result.contract.ActivityResultContracts
+import app.seb3thehacker.gearslip.audio.AudioCaptureService
+import app.seb3thehacker.gearslip.audio.CarAudio
 import app.seb3thehacker.gearslip.car.CarEnvironment
 import app.seb3thehacker.gearslip.car.CarSettings
 import app.seb3thehacker.gearslip.car.CarUi
@@ -31,6 +34,28 @@ class CarPreviewActivity : ComponentActivity() {
     private var reader: ImageReader? = null
     @Volatile private var latest: Bitmap? = null
 
+    private val audioPermission = registerForActivityResult(ActivityResultContracts.RequestPermission()) { granted ->
+        if (granted) askAudioCapture() else CarAudio.denied()
+    }
+
+    private val audioConsent = registerForActivityResult(
+        androidx.activity.result.contract.ActivityResultContracts.StartActivityForResult(),
+    ) { result ->
+        val data = result.data
+        if (result.resultCode == RESULT_OK && data != null) {
+            AudioCaptureService.start(this, result.resultCode, data)
+        } else {
+            GearslipLog.w("preview: audio capture consent declined")
+            CarAudio.denied()
+        }
+    }
+
+    private fun askAudioCapture() {
+        val manager = getSystemService(android.media.projection.MediaProjectionManager::class.java)
+        runCatching { audioConsent.launch(manager.createScreenCaptureIntent()) }
+            .onFailure { GearslipLog.e("preview: could not ask for audio capture", it); CarAudio.denied() }
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         GearslipLog.init(this)
@@ -58,6 +83,22 @@ class CarPreviewActivity : ComponentActivity() {
         val p = ScreenProjector(this).also { projector = it }
         p.start(imageReader.surface, w, h, dpi) { CarUi() }
 
+        // Stand in for the car's media sink, so the phone half of the audio path runs for real.
+        if (intent.getBooleanExtra("audio", false)) {
+            CarAudio.onConsentRequested = {
+                runOnUiThread {
+                    if (checkSelfPermission(android.Manifest.permission.RECORD_AUDIO) ==
+                        android.content.pm.PackageManager.PERMISSION_GRANTED
+                    ) {
+                        askAudioCapture()
+                    } else {
+                        audioPermission.launch(android.Manifest.permission.RECORD_AUDIO)
+                    }
+                }
+            }
+            CarAudio.attach(BenchAudioSink())
+        }
+
         GearslipLog.i("preview: displayId=${p.displayId}")
         if (intent.getBooleanExtra("hide", false)) p.setOverlayVisible(false)
 
@@ -123,6 +164,8 @@ class CarPreviewActivity : ComponentActivity() {
         super.onDestroy()
         projector?.stop()
         reader?.close()
+        CarAudio.onConsentRequested = null
+        CarAudio.release()
         CarEnvironment.stop()
     }
 }