Taskbar fixes when apps move to desktop and back

Fixes to taskbar state when tasks are moved to desktop and fullscreen.

When freeform tasks are visible and we start a new task from taskbar, it
first launches in fullscreen. Transition handler in WMSHell switches it
to freeform as needed. This switch causes launcher activity to be paused
and then resumed. Resuming launcher activity while freeform tasks are
visible, puts the launcher into an incorrect state. (Launcher should
appear paused while freeform tasks are visible).

When a freeform task switches to fullscreen, freeform tasks are no
longer visible. This causes us to resume the launcher activity. We
should not do that if a fullscreen task is visible as it puts the
launcher into an incorrect state. It appears resumed even though the
activity is paused. Added a check to DesktopVisibilityController that
checks if the activity is actually resumed before marking it appear as
resumed.

Test: manual, switch a task to freeform, launch another freeform task
  from taskbar, taskbar should continue to be available
Test: manual, switch a task to freeform, switch the task back to
  fullscreen, taskbar should continue to be available
Bug: 261234278

Change-Id: Ia8d208619fabfcc9bffff6d8d227b236cb62a00c