MPEG4Writer: Update esds atom in mpeg4 header with actual bitrate

- Calculated the average bitrate and replaced the original hardcoded value
  in esds atom with this
- Stored the max bitrate into mMeta with the key kKeyBitRate
- Replaced the original max bitrate in the esds atom with the one from the
  key kKeyBitRate

CRs-Fixed: 304561
(cherry picked from commit 55800415e9192d9a761109d543e343206141b0d3)

Change-Id: Id3973cce02be764e2f7a01ceb018037e7745125a