Fix format for VR virtual display backing buffers

Change the VR virtual display to RGBA_8888 since that's the expected
value. This also fixes allocation errors, since previously it was trying
to allocate a BLOB buffer with a height != 1.

Bug: 36990708
Test: Ran on device with vr_virtualdisplay property set to true and
entered VR.

Change-Id: I1f7c1ac90cb7403b0610e31a840224f511d53eea