Mpeg4Writer/MediaMuxer:allow empty tracks

1) Don't count tracks with no samples as malformed for MediaMuxer, but
keep the old behavior as it is for MediaRecorder.
2) When there are no samples to be written in a track, skip that one, but
compose mpeg4 file with all other tracks in it.
3) Allow notifications in MediaMuxer during stop() process.

Bug: 144108285
Bug: 146423844
Bug: 148754639

Test: 1) Unit tested by adding a video and an audio track, once leaving
      audio track with no samples and once leaving video with no
      samples.  Mpeg4 file with expected track was created both the times
      and played well.
      2) android.mediav2.cts.MuxerUnitTest$TestApi#testSimpleStartStopMuxer
      3) android.media.cts.MediaMuxerTest
      4) android.media.cts.MediaRecorderTest

Change-Id: If76a1f3b60d09836d53bce6f6e759e6a751f5538
Merged-In: If76a1f3b60d09836d53bce6f6e759e6a751f5538
(cherry picked from commit afc9f272)