Allow VirtualDisplay's to use hardware composer when in VR mode even if

physical composer does not support them.

Bug: 36071574
Test: Applied remaining wip patches, launched 2D activity while in
persistent VR mode. Observed 2D activity rendered in VR in VirtualDisplay.
Further, instrumented SurfaceFlinger code to ensure DisplayDevice's are
created and destroyed with proper composers during VR mode transition.

Change-Id: I12b09f1209fcf33f97abfd77e14c6ce61f45a5e9