Fixing issue with positioning tasks when moving to fullscreen.

- Previously, this code was incorrectly moving things to the top of the
  stack, though the stack was never moved to the front so it was never
  visible.  But as a result, testDisallowMultipleTasksInPinnedStack()
  was failing if there was already something in the fullscreen stack.

Bug: 36844394
Test: android.server.cts.ActivityManagerPinnedStackTests
Change-Id: I5108bf72b8760b187d0c92ec673bbe5cf9527e90