Squash commits of the following patches, cherry-picked from other branch - do not merge.

o Prepare for publishing MediaMetadataRetriever as public API
  step one:
  o replaced captureFrame with getFrameAtTime
  o removed getMode

o Replace MediaMetadataRetriever.captureFrame() with MediaMetadataRetriever.getFrameAtTime()
  as part of the preparation for publishing MediaMetadataRetriever as public Java API

o Remove captureFrame from MediaMetadataRetriever.java class
  It has been replaced by getFrameAtTime() method

o Replace extractAlbumArt() with getEmbeddedPicture() in MediaMetadataRetriever.java

o Publish MediaMetadataRetriever.java as public API
  o Removed setMode() methods and related mode constants
  o Removed some of the unused the metadata keys
  o Updated the javadoc
  o part of a multi-project change.

bug - 3309041

Change-Id: I2efb6e8b8d52897186b016cb4efda6862f5584c4