Use ParceledListSlice in MediaSessionManager#getSession2Tokens

Bug: 126474946
Test: atest CtsMediaTestCases:android.media.cts.MediaSession2Test;
      atest CtsMediaTestCases:android.media.cts.MediaController2Test;
      atest CtsMediaTestCases:android.media.cts.MediaSession2ServiceTest;
      atest CtsMediaTestCases:android.media.cts.MediaSessionManagerTest;
Change-Id: Ia364853d6cf638ab7128c64693b6a8cc6063036a