Desks: Additional back-nav considerations

Handles the case where a back nav transition does not contain a trigger
task and thus the forced-minimization (incl. the reparent to the
minimization root) isn't handled by
DesktopTasksController#handleRequest. Instead,
DesktopBackNavTransitionObserver checks for to-back changes and applies
the reparenting if needed.

Close transitions don't need this treatment, since there is nothing to
reparent and will just be saved as repository minimized state.

Also makes a few changes to "in desktop" and "is desktop task" logic, to
account for cases where we might not be "in desktop" anymore (because
the back nav was on the last task that also deactivated the desk) and
cases where the desktop task may not be W_M_FREEFORM.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 413024179
Test: back nav different apps (Gmail, Chrome, YT - since some generate
to-back vs close transitions) in different scenarios:
- When the app is the last task
- When the app is not the last task, and there are other tasks expanded
- When the app is not the last task, but other tasks are all minimized
Verify animation looks ok, and swiping up to overview doesn't show the
app in the desktop tile (w/o exploded view) nor as a fullscreen tile.
Test: to test non-trigger task cases - disable the back-nav handling
logic in DesktopTasksController first.

Change-Id: I95fdfbc63be77466677fc219a5ff4bddfce4f271