vrwm: Fix flickering when entering/leaving VR

There are two types of undefined layers now, 0 is used by layers
SurfaceFlinger inserts and -1 is used by Java when it does not
explicitly assign a type. Previously they were both using 0 but
have been separated to help the show/hide logic in the future.

Bug: 35915107
Test: Enter/exit VR apps, verify vrwm flicking is gone
Change-Id: I000a627f269c6533aedde1528329c04ab9db5372