hwc: Add support for Secure display

- Identify secure display layer in the hwc_list
- Need to set both SECURE_OVERLAY and SECURE_DISPLAY flags for the secure
  display layer
- Disable idle timeout for secure display usecase, as GPU should not be
  accessing secure display layer

Change-Id: I555910db77c466b5e103b24b4f0ec7f47bb210a5
Add GRALLOC_USAGE_PRIVATE_INTERNAL_ONLY definition