HLS Video Downloads

[bengelhaupt]

This is a first draft of the HLS video downloading and playback feature.

Main changes:

• HLS video streams can be downloaded with subtitles embedded
• Downloaded HLS streams can be played
• The default download quality can be selected in settings
• MP4 downloading functionality has been completely removed; MP4 streams are only fallback for online playback
• Interfaces and behavior of file and video download have been generalized and unified (notifications etc.)
• SD card support remains, but storage migration (moving data between internal storage and sdcard) had to be removed because it is not compatible with the way ExoPlayer stores the video data
• Many under-the-hood changes for download handling (Downloads are now always handled through their respective Downloaders. That e.g. means that a file download must not be removed manually by simply deleting the file. Our Download UI takes care of that.)

TODOs:

• HLS video size estimation (no video size is available)
• HLS casting (excluded from requirements)
• Dialog with hint on first download, that the video quality can be changed in settings
• documentation
• unit + instrumented tests
• testing on physical devices
• how to migrate existing downloads?
• Seek preview does not work for downloaded HLS streams

Fixes #212