gralloc: Add GRALLOC_PRIVATE_USAGE_WFD flag

1. Screenrecord and WFD use virtual display and encoder.
   Since HAL supports only one virtual display, we need a
   definite way to distinguish both use-cases.

   Failure to differentiate will break Screen record + WFD
   concurrent usecase.

   WFD stack will set this usage flag and SF will query the usage
   flags to differentiate WFD and screenrecord usecase.

2. Deprecate GRALLOC_USAGE_PRIVATE_EXTERNAL_CC and
   GRALLOC_USAGE_PRIVATE_EXTERNAL_BLOCK flags as these are no
   longer used.

Conflicts:
        libhwcomposer/hwc_utils.h

Change-Id: Ifa260c5af2371eea830744ce62a5a83b8cfd0de7

Conflicts:
	libhwcomposer/hwc_utils.h
	msm8660/libgralloc/gpu.cpp
	msm8660/libgralloc/gralloc_priv.h