VR: Create VR implementation for HWC HIDL interface

* Implement the IComposerClient interface for a VR HWC.
* Introduce new command to send layer information to the VR HWC.
* Add observer interface in the VR HWC service allowing VR Window
  Manager to consume SurfaceFlinger output.

Bug: 32541196, 33297385
Test: Compiled and ran with modified surfaceflinger that connects to the
VR hwcomposer and made sure there are no crashes.

Change-Id: Ic2c882ba5e4c570cbd0b7a180845a5e61a768cad