back buttons on every screen, exit to the car's own UI, smoother link

Every screen reached from Home has a themed back button in its header. A new Exit tile
asks the car to show its own interface, as Android Auto's does. All writes to the car
now go through one thread, so reading never waits on a write; LIVI no longer stalls
for two seconds at a time. Touches go to the front of the main thread's queue.

gearslip/src/main/java/app/seb3thehacker/gearslip/car/CarSettingsScreen.kt

@@ -74,7 +74,7 @@ fun CarSettingsScreen() {
             .padding(horizontal = 24.dp, vertical = 16.dp),
         verticalArrangement = Arrangement.spacedBy(20.dp),
     ) {
-        Text("Settings", style = MaterialTheme.typography.headlineMedium, fontWeight = FontWeight.Bold)
+        ScreenHeader("Settings")
 
         ChoiceRow(
             "Open on connect",