Refactoring how the box table entries are stored in MPEG4Writer

o Helps drastically reduce the time writing all the box table entries for long duration video recording sessions.
  Experiments showed that the time is reduced from multiple seconds to less than one second for around 2.5 hours of video recording.

Change-Id: I6a696823e2938b76fcf2c73605f51bf5e5c2d749

related-to-bug: 6435176