SurfaceView: Check ViewRootImpl non-null

This call to updateRelativeZ may be triggered from the
RT frame callback which may be triggered after we are detached
from the Window and in that case will be null. If we are detached
we are also going invisible so there is no need
to set a relativeZ.

Bug: 158706756
Test: Existing tests pass
Change-Id: I46aa824807b7b275e6a015c428fe7467a72ca949