MPEG4Extractor:seek with empty edit list entry

Among the many changes that went in as part of CL ag/9501009, reading empty edit list entry was
also added in MPEG4Extractor.  Code modifications for seek functionality with the same is done
now.

Bug: 150956121

Test: atest android.media.cts.MediaMetadataRetrieverTest
      atest android.media.cts.MediaMuxerTest
      atest android.media.cts.MediaExtractorTest
      atest android.media.cts.MediaRecorderTest

Change-Id: I3ec065f56259eaf1d923c6701713c6a445e4e2f4