Fix sysui crash with bubble to fullscreen flag

When bubble to fullscreen flag is enabled we create a DesktopTasksController, which depends on a DesksTransitionObserver.

However, the Provides method for DesksTransitionObserver doesn't check
for the state of the bubble to fullscreen flag so it may be empty, which
results in a crash on startup.

This change updates the Provides method for DesksTransitionObserver to
interrogate the state of the flag.

Flag: com.android.wm.shell.enable_bubble_to_fullscreen
Fixes: 401021955
Test: manual
        - enable bubble to fullscreen flag
        - verify sysui doesn't crash on startup

Change-Id: Ia6b8b0c1643e443cb6a414621c1e478aa88b1040