In H264, there may be multiple consecutive video slice NAL units in the same frame. The original code assigns a new access unit for every video slice NAL unit, which is incorrect. Fixes #134. Change-Id: I4d44271df48cb08867ddd02f7494fb3573af3356
Also move codecs related classes in mp4 to codecs directory. Change-Id: I83db2bcf9e66b405fcceed78918b762b26cd7fce