Hide constructor of MediaController#PlaybackInfo

This CL reverts the API change introduced by
"1dfec66d: Make hidden constructor of PlaybackInfo as @SystemApi"

Bug: 128245348
Test: atest CtsMediaTestCases:android.media.cts.MediaSessionTest;
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest;
      atest CtsMediaTestCases:android.media.cts.MediaBrowserTest;
Change-Id: I6deeb048d6d209baa085792edf5ef2fdd8bb1d7c