Gallery2: support loop/single video play mode

This change allows a video to be played repeatedly. It also
introduces an extension framework.

Change-Id: I5566192f138c1f0fd889b85496dd27fbf2aed10d
CRs-Fixed: 507973
Gallery2: support stereo/single track play mode

(cherry picked new files from commit id
990c6d43ea7c184846e19d41bef0d93aee4e581b)

Change-Id: I2d7d523ca586a031c3fb139eda321d5073b30589