SDK / exoplayer

Discontinuity reasons may not be precisely obtained for remote
player. 'Remote Player' means that playback is owned by another
app or device and the same playback can be controller by other
clients simultaneously. The MediaController is an example.
If the remote playback doesn't provide discontinuity reason, then
player cannot differentiate between automatic playback transition
and seekTo() from another client.

This CL tweaks the discontinuity reason definitions, so reasons
can be obtained without remote playback's support.
This doesn't effect the local Players, such as SimpleExoPlayer.

PiperOrigin-RevId: 368579577