Unhide MediaPlayer#MEDIA_INFO_STARTED_AS_NEXT

Expose a listener constant which is used with OnInfoListener() in
order to notify that the player instance started the playback since
previous player just completed the playback.

Bug: 76121592
Test: make offline-sdk-docs
Change-Id: I69ade8e9f6916bc653ea2831e3721dc7d171dc0e