update checker: finds new releases, downloads and installs them

Checks GitHub's latest release once a day when the app opens and after a
drive. A new version shows as a card on Home, a notification after the
drive, and a card on the car screen at plug-in while parked. Downloads are
checked against GitHub's SHA-256 and never install while the car is
connected. Update notifications can be turned off in Settings.

gearslip/src/main/AndroidManifest.xml

@@ -4,6 +4,7 @@
 
     <!-- The projected WebView loads real pages; without this every load fails silently. -->
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <!--
       Phone mirroring. Android refuses to hand over a MediaProjection unless a foreground