This helps to highlight and document new requirements for apps,
e.g. to add the FOREGROUND_SERVICE_MEDIA_PLAYBACK permission for
background playback.
Issue: google/ExoPlayer#11239
PiperOrigin-RevId: 548666056
Basically this change removes a bug that makes video playback stuck when
a video is playing and the user taps the UMO notification to get to
the player activity.
- Use `launchMode="singleTop"` for `PlayerActivity`
- Change session activity to a back stacked activity on service `onDestroy`.
Using a back stacked activity `onDestroy()` will be useful once this demo
app implements playback resumption.
The rest of the changes are aesthetic:
- clean up and optimize screen space usage in UI of `PlayerActivity`
- changed some colors, paddings and spacings
- adds a default artwork for the `PlayerView`
PiperOrigin-RevId: 534152052
