Clear last focus of previous display when reparenting

Assume 2 displays:
 D0 contains activities A, X
 D1 contains an activity B

The update order of focus is from top to bottom. When reparenting
X from D0 to D1, and D1 becomes the top display:
 1) D1: X gains focus, B loses focus
 2) D0: A gains focus, X loses focus
That results the top activity X on top display 0 does not have focus
(the state in client side view root).

Bug: 119664976
Test: atest DisplayContentTests#testClearLastFocusWhenReparentingFocusedWindow
Test: atest WindowFocusTests
Change-Id: I5bf3546cde3ac6d4be45d1a971cda033663cc919