Add support for different types of external-only layers
EXTERNAL_ONLY: General purpose
EXTERNAL_BLOCK: Blocks the display of other ext-only layers
EXTERNAL_CC: Used for closed captions with videos.
Change-Id: I9cc7b9a1dbf1085194725474d6a4d1094e27a0c8
Acked-by: Saurabh Shah <saurshah@codeaurora.org>
