MPEG4Writer:Correct track duration BFrames muxing

1. Subtract start time offset from duration of tracks while Bframes muxing
2. Include start time offset while calculating segment_duration for elst
entry.

Bug: 123093098

Test: 1) android.media.cts.MediaMuxerTest
      2) cts-tradefed run cts-dev -m CtsMediaTestCases --compatibility:module-arg \
         CtsMediaTestCases:include-annotation: android.platform.test.annotations.RequiresDevice

Change-Id: I4b5375c3f01c432dbf59e3c34632fa7d0fa66a68