Fix gradle leanback build by switching to androidx leanback.
Fix gradle leanback build by switching to androidx leanback.

PiperOrigin-RevId: 237467628

google/ExoPlayer,ened/ExoPlayer,superbderrick/ExoPlayer,androidx/media,stari4ek/ExoPlayer,superbderrick/ExoPlayer,androidx/media,google/ExoPlayer,amzn/exoplayer-amazon-port,ened/ExoPlayer,saki4510t/ExoPlayer,saki4510t/ExoPlayer,stari4ek/ExoPlayer,amzn/exoplayer-amazon-port,amzn/exoplayer-amazon-port,superbderrick/ExoPlayer,saki4510t/ExoPlayer,ened/ExoPlayer,stari4ek/ExoPlayer,androidx/media,google/ExoPlayer