libgralloc: Add support to use CAMERA_HEAP

- Adds flag GRALLOC_USAGE_PRIVATE_CAMERA_HEAP to use the dedicated
  CAMERA_HEAP
- Remove GRALLOC_USAGE_PRIVATE_CP_BUFFER which is not used anymore

Change-Id: Iefea88d9a67d2fa0ca74492f508e1d5508d08983