Set 90% task to be focused afer divider swap

* In this CL (ag/28401411), we ensured that during the
divider swap animation the split root window isn't focusable
because of the edge case where the user kept double tapping
the divider, one of the taps would go to one of the swapping apps mid
animation and cause issues.
* After the animation is done we re-enable focus to the root window
* Now, when we request move to top to the 90% app, we have to make sure
it happens after focus has been granted otherwise WM ignores our call
(see the isFocusable() check in ActivityRecord#moveFocusableActivityToTop())
* There's still an ever so slight race condition where if the window isn't
fully focused but user does the back gesture it can go to the wrong (10%) app,
but hopefully this is sufficient enough for now

Test: Manual
Flag: com.android.wm.shell.enable_flexible_two_app_split
Fixes: 416056529
Change-Id: Ifcb126d197891134d09fe7e15b17855a8b5094e2