Drop the pointer-down-outside-focus event

... if it focused on an Activity that's entirely occluded by
the task top activity.

We don't check the mVisibleRequested state because the activity
behind may still be visible due to the shared-element transition.

Bug: 360385918
Test: Explicitly delay ON_POINTER_DOWN_OUTSIDE_FOCUS 300 ms
Flag: EXEMPT bugfix
Change-Id: I5871ea8cae940842ca57294affc76b485e8e1365