Desks: Exclude desk roots when looking for focused desktop tasks

Prior to desktop tasks being organized inside a desk root task, using
getRunningTasks with a W_M_FREEFORM was an ok way to identify desktop
tasks. However, now that the desk root itself is also a running freeform
task, and considering desktop tasks may also soon be W_M_FULLSCREEN, it
is better to check the repository for whether a task is a desktop task,
regardless of its windowing mode.

Also fixes an issue where FocusTransitionObserver considers the desk
root task the "focused in display" task even if it contains focused
children by:
1) Filtering for leaf tasks
2) Having shell transitions report children as FLAG_MOVED_TO_TOP even if
   they had already been reported to top, as long as their parent is
   also to-top and had not been reported before. This fixes an edge case
   where reparenting an already to-top fullscreen task to a desk that is
   being brought to front reported only the desk as MOVE_TO_TOP, but the
   child change was not MOVE_TO_TOP, resulting in
   FocusTransitionObserver not considering the child as a focus
   candidate.

Flag: com.android.window.flags.exclude_desk_roots_from_desktop_tasks
Fix: 417876539
Bug: 405354639
Fix: 413127211
Test: use keyboard shorcut to move task from desk->fullscreen, verify
it's the child task that moves to fullscreen, not the desk task
Test: same as above, but for a move from desk->split or tiling
Test: Open fullscreen task and move to desk with handle menu button -
verify the App Header color is that of focused-in-display tasks (more
opaque)

Change-Id: Ice8a0af90d7a01b3f9a475660053d99a79fa2648