Remove desktop tasks on back navigation.

This change makes sure that when we get closing type transition(CLOSE or
TO_BACK), the trigger task is removed. As a result is task is removed
from recents. Initial manual testing showed the approach as sound but we
will be monitoring during flag ramp up to make sure we don't miss any
edge cases.

For the tests, altered the current tests to test the following 3
conditions:
- Wallpaper disabled, back nav disabled
- Wallpaper enabled, back nav disabled
- Wallpaper enabled, back nav enabled

Bug; 352266521
Test: atest DesktopTasksControllerTest
Flag: com.android.window.flags.enable_desktop_windowing_back_navigation

Change-Id: I6cccdb9a0c0d31e34f671b9484cc37804fba6fd2