Fix task movement if top running activity of focused stack is null

If the target task is non-null and current top task is null (e.g.
the activity may be finishing or use FLAG_ACTIVITY_PREVIOUS_IS_TOP),
we should still consider they are different task so the target task
can be moved to front instead of only move its stack.

Bug: 131589476
Test: atest ActivityStarterTests#testBringTaskToFrontWhenFocusedStackIsFinising
Change-Id: Ibf8befae74aab9e308d82d2149c41cd069615988