General

• Updated ExoPlayer to 2.9.1 which includes TransferListener
• Added a getSelectedTrackIndex method that takes the groupIndex
• Addresses #658 (Adds getSelectedTrackIndex to the VideoView and AudioPlayer)
• Addresses #666 (Allow for player to re-select a default track)

Bugs

• Fixed #541 (Track mapping index calculation was still off in some cases)
• Fixed #663 (Leanback controls not shown when HW FF or RW was clicked)