Straw phase M-2: MediaSession + audio focus + service skeleton

Wraps the existing Activity-owned ExoPlayer in a Media3 MediaSession. Side
effects:
- Lock-screen media controls (play/pause)
- System media notification with play/pause buttons
- Audio focus + ducking (other audio dims when straw plays)
- Bluetooth + headset hardware-button routing
- Plays nicely alongside other media apps' notifications

The ExoPlayer is also configured with AudioAttributes (USAGE_MEDIA +
CONTENT_TYPE_MOVIE) and handleAudioFocus=true so other apps can request
focus correctly (e.g., a phone call ducks/pauses straw).

Also scaffolded but NOT yet wired:
- PlaybackService extending MediaSessionService — the foundation for
  true background-after-Activity-kill audio. Manifest entry + deps
  added (media3-session 1.4.1, concurrent-futures-ktx 1.2.0). The
  Activity-to-Service migration via MediaController is M-3 work.
- POST_NOTIFICATIONS, FOREGROUND_SERVICE,
  FOREGROUND_SERVICE_MEDIA_PLAYBACK, WAKE_LOCK permissions in manifest.

For the user right now: lock the phone while a video plays — media
controls appear on the lock screen. Open another app — notification with
play/pause sits in the shade. Press a headset's play/pause — straw
responds. Pull the home screen — PiP keeps the video floating + audio
continues. Full screen-off-background-audio after killing the activity
arrives in M-3.

strawApp/build.gradle.kts

@@ -106,4 +106,8 @@ dependencies {
     implementation("androidx.media3:media3-exoplayer-dash:1.4.1")
     implementation("androidx.media3:media3-exoplayer-hls:1.4.1")
     implementation("androidx.media3:media3-ui:1.4.1")
+    // Media3 session — MediaSessionService for background audio + lock-screen controls.
+    implementation("androidx.media3:media3-session:1.4.1")
+    // Guava ListenableFuture support for awaiting MediaController connect.
+    implementation("androidx.concurrent:concurrent-futures-ktx:1.2.0")
 }