hwcomposer: Add HWC_BLIT compositiontype value

- HWC_BLIT this layer will be handled in the HWC, using a blit engine
- setting this will inform Surfaceflinger to flip FB_TARGET

Change-Id: I25d19ec41b57863d451c3e390109c912faf1381c

Signed-off-by: Nathan Chan <e334cn@gmail.com>