App 2.6.0: floating button to open the app

The MG4's launcher does not list third-party apps, so the app could
only be opened by reinstalling it. Now a small round MG-red button
floats over the car's screens: tap opens the app, drag moves it (the
position is kept and clamped to the screen), and it hides while the
app itself is open. A switch in the Upload tab turns it off.

The always-on service owns the overlay, so the button is there from
boot. It needs the overlay permission (SYSTEM_ALERT_WINDOW), which the
car build has through the system UID; without it, or if the window
manager refuses, the button stays away and nothing crashes, and the
Upload tab offers the system permission screen.

Tested on an Android 9 AOSP emulator at the head unit's 1920x1080 /
160 dpi with the car APK (installs as UID 1000): button appears over
the home screen, tap opens the app, hidden while open, drag moves it
without opening the app, position survives a force-stop, the switch
works through the UI. FloatingButtonGeometry has 3 unit tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QLJfZQZeAKAwSqViTmLzki

app/src/main/AndroidManifest.xml

@@ -13,6 +13,8 @@
     <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_LOCATION" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
+    <!-- The floating button that opens the app; the head unit's launcher does not list it. -->
+    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />
 
     <!-- Wi-Fi auto-join. CHANGE_WIFI_STATE covers enabling the radio and asking the
          supplicant to join a network the unit already has saved; ACCESS_WIFI_STATE covers