MediaPlayer subtitle data / time discontinuity interfaces

Expose in SDK the interfaces necessary to implement unbundled
  subtitle support, which requires:
  - exposing an interface to receive the encoded subtitle data
  - exposing an interface while allows a renderer to synchronize
    its rendering clock with the media time of the player.
Deprecate duplicated timed text constant, all timed text / sub
  mime types are defined in MediaFormat.

Bug: 76121592
Test: make offline-sdk-docs
Change-Id: I42cc1d69069babb69a2feeba3f54da89889f4602