feat(ringtones): let a music app play the alarm

"Choose from a music app…" in the sound picker browses any app that shares
its library through Android's MediaBrowserService and stores the choice as a
clockula://media URI in the existing ringtone field — no schema change, no
new permission, no internet. The music app streams with its own.

At ring time the app is asked to play through its media session. Every wait
is bounded and any failure falls back to the ordinary alarm sound: the app
won't connect or start, stalls or buffers too long, or the media stream
might not be heard (headphones or a classic Bluetooth device connected, Do
Not Disturb muting media). The ramp drives the media stream and the user's
media volume is restored when the last media ring ends.

RingAudioPlayer.start no longer suspends: vibration starts up front, the
timer collector is not held up, and a generation token keeps late work from
reviving a stopped ring.

app/src/main/AndroidManifest.xml

@@ -50,6 +50,16 @@
     <!-- The alarm vibrates, and vibrates on its own when no audio source opens. -->
     <uses-permission android:name="android.permission.VIBRATE" />
 
+    <!-- "Choose from a music app…": package visibility for the apps that share
+         a library through Android's standard MediaBrowserService, and for
+         nothing else. A visibility declaration, not a permission — and no
+         network: the music app streams with its own. -->
+    <queries>
+        <intent>
+            <action android:name="android.media.browse.MediaBrowserService" />
+        </intent>
+    </queries>
+
     <application
         android:name=".ClockulaApp"
         android:allowBackup="true"