Make Session2Token#getExtras @NonNull

Bug: 128413976
Test: atest CtsMediaTestCases:android.media.cts.MediaSession2Test;
      atest CtsMediaTestCases:android.media.cts.MediaController2Test;
      atest CtsMediaTestCases:android.media.cts.MediaSession2ServiceTest;
Change-Id: I007efc4318d2bbf0e3849cc00f22ff9dac9784da
3 files changed