PlayerNotificationManager: support custom queue navigation

### [REQUIRED] Use case description

I have a custom queue navigator not based on a TimelineQueueNavigator. Inside a media session it works fine, however integration with the PlayerNotificationManager is more cumbersome than necessary. I tried to create a subclass from PlayerNotificationManager but the next() and previous() methods are private and most of the instance variables, so additional work is required. Copy and paste of the PlayerNotificationManager to create a custom version should not be easier than reusing the existing one. It does almost everything I want.

### Proposed solution

Enable delegation for queue actions to a custom QueueNavigator (same one as in the mediasession extension) so the onSkipToPrevious and onSkipToNext can be reused. In general is would be awesome to have less objects "private" and be able to supply custom versions easier.

### Alternatives considered
Complete custom management of notifications based on the media session.