Gallery2: support audio effects during video play.

Porting from kk with fixes

Conflicts:
	res/values/strings.xml

Change-Id: I267c3c7e402fd5cf2bb223547003af491dd8289e
Signed-off-by: Xiaojing Zhang <zhangx@codeaurora.org>
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