Register FocusTransitionObserver.dump through ShellCommandHandler

Currently, FocusTransitionObserver.dump is registered through
DesktopTasksController only when desktop mode is supported.
However, FocusTransitionObserver exists independently of desktop mode,
so the registration should be done directly through ShellCommandHandler.

Bug: 408122780
Test: atest FocusTransitionObserverTest
Test: adb shell dumpsys activity service SystemUIService WMShell
Flag: EXEMPT DEBUG

Change-Id: I1d874a1f4ac09e2ab738e6be9716450433998f03