Displays that have focused apps can also be top focused

There was no focus window during activity switches.
- While starting an activity, the previous top activity was paused and set
  the AppWindowToken.hiddenRequested to true (while #setVisibility)
- While finishing current top activity, the focused window was updated a
  bit early, which was before the next top activity resumed.

In both two cases, the focused window was set to null for a small amount of
time. So, we tried to look up the focus in other displays and resulted
the extra onWindowFocusChanged() calls for activities on other displays.

Bug: 131374329
Test: DisplayContentTests
Change-Id: Ia77bf697c237696cad7b42ca6b38157eff497b23