Clear mPreferredTopFocusableRootTask when root becomes empty

In multi-desks, the focused root task may have its only child desktop
window reparented out into the back of another container, leaving the
empty root incorrectly set as focused. Clear the TDA preferred focusable
root task when this happens so that getFocusedRootTask() doesn't select
the empty task as the focused one.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 420858253
Test: minimize the last task in a desk, then check dumpsys:
 1) mPreferredTopFocusableRootTask is the DesktopWallpaperActivity task
 2) Desk remains "active" in DesktopRepository and DesksOrganizer
Test: back-nav the last task in a desk (Gmail log in screen to force a
move to back instead a of a close) - verify the same as above

Change-Id: I0e7fb75a62bb8f8d1444e88e95904ba7224cc354