gralloc: Add GRALLOC_PRIVATE_USAGE_WFD flag
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.