MusicDao: UI improvements, ExoPlayer integration, streaming improvements

[Tim-W]

Related to https://github.com/Tribler/tribler/issues/5134

• Implement ExoPlayer mediaplayer for stability in listening to tracks while they are downloading (which is used in major apps like youtube)
• Prioritize torrent pieces at start of selected track to buffer and play it asap. See video 1 where the user does not have to wait for full download before it can be played.
• Browse through different playlists
• Keep playlists in memory using trustchain persistant local database
• Discover playlists: when a playlist tagged in the form of a trustchain block with type "publish_release" is found, it is shown in overview
• Fixed listening to own tracks: When you submit a Release, you can play it immediately. Torrent file is saved on cache so that on next app boot the tracks can be played fast.
• Created ContentSeeder class, which, in the background, selects all torrents of which the torrent metadata is already fetched from the magnet, are all seeding. With a maximum of 10 torrents for now (lead for future experiment?)
• Show selected song description
• Pre load tracks in the background. Current selected track has highest priority to download.
• FAB button added which allows for either selecting a tracks from local folder or for pasting a magnet. All in one form/interface. See video.