Handle task closing only when we are in desktop.

We used to handle task closing outside of desktop as well. This resulted
in adding tasks into closing list even when they were outside of
desktop. This would result in empty desktop state in some cases.

Test: atest DesktopTasksControllerTest
Fix: 358552898
Flag: EXEMPT Bug fix
Change-Id: I31c1ef5108e3f400fe6518a9fe7dea1919dbd049