Improve the multi player showcase

[MGaetan89]

Pull request

Description

This PR improves the multi player showcase in the following ways:

• Only have one of the two players to play audio at a time
• Clicking on the inactive player activates it, and disables the other one
• A media notification is now shown with, linked with the current player
• Only the analytics for the currently active player are sent

Changes made

• Create a MultiPlayerViewModel to manage the players, the active state toggling, and the media notification
• Add the missing android.permission.POST_NOTIFICATIONS permission in the pillarbox-demo manifest
• Make the PlayerControls background control customisable
• Update CurrentMediaItemTracker to support restarting tracking

Checklist

• Your branch has been rebased onto the main branch.
• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• All pull request status checks pass.

[github-actions]

Code Coverage

Overall Project	55.25% -0.01%	🟢
Files changed	87.5%	🟢

Module	Coverage
:pillarbox-player	73.15% -0.04%	🟢

Files

Module	File	Coverage
:pillarbox-player	CurrentMediaItemTracker.kt	95.51% -0.45%	🟢