Bundle our custom ExoPlayer components into aar instead of directly
depending on the project. This just makes things far better regarding
ease of use and reproducible builds.
Expose a custom MediaButtonReceiver that handles the media button
intent. This is not because I wanted to implement this. Some apps
like gadgetbridge just blindly query ACTION_MEDIA_BUTTON instead of
relying on the more modern MediaController API, which I expected
most apps would use instead.
Resolves#62.
