Don't let exiting starting window control system bars

When an activity (appA) starts another activity (appB) behind a starting
window (winS), the app window (winA) of A will lose the window focus.
And then mTopFullscreenOpaqueWindowState will control the system bars
while there is no focused window. WinS might become the controlling
window temporarily before it is removed. The controlling windows could
be:

winS --> winA --> winS (exiting) --> winA --> winB

If the requested visibilities of system bars between winS and winA are
different, system bars might look flickering.

This CL is to let winA keep controlling system bars until the app window
(winB) of appB becomes the focused window. The controlling windows with
the new logic will be:

winS --> winA --> winB

Fix: 322768312
Test: Cold-start `The Spectator` on a device with a pinned taskbar and
      set animator duration scale to animation off. See if taskbar
      flashes on the starting window.
Change-Id: Ic3081d97edf755a3bd2d6ec8703f6ca42bb39dab