VR: Move VR HWC initialization in SurfaceFlingerView

Initialize the VR HWC at first use since it is also needed to register
the frame observer VR WM uses to display SurfaceFlinger output.

Note, need the real implementation, rather than the passthrough
interface received via IComposer::getService().

Bug: b/35403318
Test: Compiled
Change-Id: I7569136edcef57f545e3d1c3daf644f48fd6b907