hwc: Use correct z-order in video overlay

Set z-order of video layer to 0 and framebuffer to 1 in case
of video overlay composition so that the blending takes effect.
Also set is_fg OFF for framebuffer.

Change-Id: I05f930074d265d690f29a1003b7c28077df89bbf
CRs-fixed: 458180
5 files changed