base: Add support for FLAG_SUPPORTS_PROTECTED_BUFFERS

For the clients (WFD) which need secure HWC composition,
FLAG_SUPPORTS_PROTECTED_BUFFERS need to be set.
- Define new flag VIRTUAL_DISPLAY_SUPPORTS_PROTECTED_BUFFERS
- In VirtualDisplayAdapter, add support for the flag
FLAG_SUPPORTS_PROTECTED_BUFFERS.

CRs-Fixed: 653794
Change-Id: Ib954943f3e27067b2805188771f951c115833237