Remove MediaSessionEngine.CallbackStub

This CL also removes SessionCallbackLink.CallbackStub class
from the system API list.

Bug: 123500993
Test: make update-api -j; make -j;
      atest CtsMediaTestCases:android.media.cts.MediaSessionTest;
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest;
Change-Id: I59b4d4ce058c53c82d816152039c51b109f0c245
4 files changed