Show IME correctly when top-most display focus changed.

In previous design, the IME focus is changed when receiving
window focus change from ViewRootImpl.

For Multi-Display concept, it should be needed to aware the top
display focus changed in different displays but both are already
had the focused window without change case.

Sending REPORT_FOCUS_CHANGE when top display focus changed for
ViewRootImpl, let IMM can re-focus IME window on right display.

Bug: 117491872
Test: atest ActivityManagerMultiDisplayTests
Test: atest FrameworksServicesTests:DisplayContentTests

Change-Id: Ia46738a5da6dbc334bf937b0f6656a57523c28a7