Signal display refresh when a VR_HWC client is registered

In steady state, SurfaceFlinger will not update the surfaces. If the
observer is registered after steady state, the client will not receive
any updates. This forces SurfaceFlinger to update the display surfaces

Bug: 64207223
Test: Updated unittest
Change-Id: Ied7ffcaaacec25d36dafb948489538155a0c865f