ViewRootImpl: Update opaque flag if SurfaceControl changes from relayout

The opaque flag is necessary for the compositor to blend the layers
correctly and prevent translucent pixels from showing the layer
underneath during animations.

Test: manually punch holes in apps and verify opaque apps don't show the layer underneath
Test: automated tests to follow
Bug: 198924563
Change-Id: I1116c51b2efd4e9c0fe2acbca080291f3950245c