Add task focus change support to FocusTransitionListener

This change adds task focus change support to FocusTransitionListener
to replace TaskInfo#isFocused with system-wide focus state.

The ideal, long-term solution is to implement this with a unified task
repository in WMShell, but as the project isn't ready yet, this change
workarounds some pain points that come from the compatibility with
existing APIs.

With this change, instead of FocusTransitionObserver registering as
TransitionObserver, focus state in FocusTransitionObserver is updated
in FreeformTransitionObserver#onTransitionReady(). This is because
focus state needs to be updated before FreeformTransitionObserver
processes the transition. Having one observer should be simpler
than having a order requirement between two observers, and this is
effectively merging FocusTransitionObserver and
FreeformTransitionObserver from the perspective of TransitionObserver,
but keeps these classes seperately to both avoid circular dependency
and isolate responsibilities. (Now FocusTransitionObserver no
longer implements TransitionObserver.)

Bug: 371143601
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: atest FocusTransitionObserverTest
Change-Id: I68b6aaa44cf8578d3dcac3277c66e3d07738eef8