Report display change for display focus switch

Currently, when display focus has changed, the task that caused
the focus switch is reported with the MOVED_TO_TOP flag, and
WMShell “infers” the display focus event from it. This is a bit
tricky in some cases because even when a task gets "moved to front",
sometimes the display may not come on top. For example, when a
task gets minimized, the task just behind the originally top task
comes on top, but this doesn't involve moving the display to front.

To avoid these confusions, with this change, the display itself is
reported with the MOVED_TO_TOP flag when display focus switches,
which greatly simplifies the logic and saves us considering tricky
edge cases.

Bug: 369940439
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: TransitionTests
Test: atest FocusTransitionObserverTest
Change-Id: I61a5649cdc917a7d7a344329f247580cb4218190