Desks: Exclude non-filling tasks from show system bars policy

Generalizes the policy to keep system bars showing when the top freeform
task doesn't have fullscreen bounds to account for scenarios in which
the top freeform task has no content (i.e. is not filling), such as in
desktop windowing where desktop tasks are children of a "desk" root
task, which should not be considered as the "top freeform task".

Flag: com.android.window.flags.restrict_freeform_hidden_system_bars_to_filling_tasks
Fix: 415267515
Fix: 413139450
Test: with multiple desks enabled, open an immersive app in desktop and
verify the system bars don't hide, then make it use fullscreen bounds
and verity the system bars do hide as expected

Change-Id: I2ff05b667214d22bf571bb8f6ea669d39cc1b2c1