VR: Pass display size in the HWC observer callback

1) Unify the places where primary display size is queried.
2) Allows virtual displays to have a different size than the primary
display and the size propagates properly to VR HWC and its observer.

Bug: None
Test: Unittest and manually on device
Change-Id: I7e026c4f9b018d47ca400247beed27841d71c806