[DO NOT MERGE] Fix TaskFragmentOrganizerTest#testDeleteTaskFragment

Before, the test creates and deletes an empty TaskFragment, which may
not trigger to dispatch the pending callback event to the organizer.

Now, we launch an activity after the TaskFragment is removed to trigger
a surface placement for that.

Bug: 259360670
Test: atest CtsWindowManagerDeviceTestCases:TaskFragmentOrganizerTest
Change-Id: Icb1df0d642d83a553bee9d2527f298ffde56445f