Sysui vis needs to be recomputed in the same code path as showing/
hiding the system bar (code path = finishPostLayoutPolicyLw) so
it can perform the new fade in/fade out to transparent modes at
the correct time.
Turns out no new state tracking is required, we already keep track
of this window as mTopFullscreenOpaqueWindowState.
So prefer mFocusedWindow when computing sysui vis as before,
but if null fallback to mTopFullscreenOpaqueWindowState.
Bug:10561554
Change-Id: I492766989a67fdac4f030451dcf00f6741a556c0
79da83347a