External player: background service, MediaSession & controls

[osphvdhwj]

Adds PlayerService, MediaSession, MediaStyle notification, speed control and subtitle placeholder. TODO: integrate extractor for YouTube streams, subtitle track support, casting, DRM and UX polish.

What's new in this PR:

• Foreground PlayerService: Background playback with persistent MediaStyle notification
• MediaSession integration: Lockscreen and system playback controls work properly
• Service-based architecture: Activity delegates playback to service for better lifecycle behavior
• Speed control: Button to cycle through playback speeds (1.0x, 1.25x, 1.5x, 2.0x, 0.5x)
• Subtitle toggle: UI placeholder for future subtitle implementation
• Updated manifest: Added FOREGROUND_SERVICE permission

Technical details:

• PlayerService: LifecycleService with ExoPlayer and MediaSession integration
• MediaNotificationManager: Handles notification creation and media controls
• ExternalPlayerActivity: Updated with service integration and new UI controls
• Enhanced layout with speed and subtitle controls

Testing:

1. Build and install the app
2. Test with direct video URLs: adb shell am start -a android.intent.action.VIEW -d "https://example.com/video.mp4" org.newpipe.externalplayer/.ExternalPlayerActivity
3. Verify background playback continues when pressing Home
4. Test notification controls (play/pause/stop)
5. Try speed cycling and subtitle toggle

Next steps:

• Integrate with NewPipe's extractor for YouTube stream resolution
• Implement actual subtitle support in PlayerService
• Add proper audio focus handling
• Add instrumentation tests for service lifecycle

[TobiGr]

What's the goal of this PR? I don't really understand what you are trying to achive and why you are deleting a lot of code. Please explain what this PR is trying to achive.
Also please not that new features should only be introduced on the refactor branch and not dev. There is also development on our new player library NewPlayer.