Ability to play local files (also for tests)

[Stypox]

It would be cool if the player could play local files because:

• maybe in the future we might want to add a "local-filesystem" service to NewPipe?
• it would be very useful to make integration tests where many actions happen, e.g.:
  • user adds video to queue from menu, then closes app, then adds another video from RouterActivity/sharemenu, then pauses, goes to next video and back prev, ...
  • then we want to check which video is currently selected, which progress it has, ...
  • this would allow defining rigorously how some actions should behave (e.g. should going to next and then back to prev restore the progress, or start anew from 0:00?)

Some considerations:

• there are way too many file formats and codecs to be able to play them all, so we should focus just on formats and codecs directly supported by ExoPlayer
• for tests an even more basic set of supported formats would suffice (e.g. only .webm)

[theScrabi]

This should already be possible. When implementing MediaRepository it might look like its not, because an HttpDataSourceFactory is required. However, just because such a factory needs to be supplied it does not mean it must be used. It should be easily possible to supply NewPlayer with local media files coming from the StorrageFramework, a Unix file descriptor or simple Unix file system.

[AudricV]

When implementing MediaRepository it might look like its not, because an HttpDataSourceFactory is required.

Supporting a generic DataSource.Factory in MediaRepository is the best option we have. We would then just need a FileDataSource.Factory and pass a local URI in a Media3 MediaItem. The first part is done in #7.