Add support for external-only display. Layer buffers
marked with GRALLOC_USAGE_EXTERNAL_ONLY will be
displayed on external panel only, if available.
In addition if the layer buffer also has the flag
GRALLOC_USAGE_EXTERNAL_BLOCK then only that layer
will be displayed on external (During suspend).
When this mode is on, the default mirror mode for
external is stopped.
Conflicts:
libhwcomposer/hwcomposer.cpp
Change-Id: I8ea2920384c5b68311ee3fe00707d01ab6568b52
