Only need to dismiss stage if one of the task is closed.

If both stages are closed, it might be starting a new full screen task,
so don't handle it.

Bug: 279117417
Test: no regression on 278060807.
Test: atest WMShellFlickerTests:SwitchBackToSplitFromAnotherApp
Change-Id: I09be7fafff4286493bc1456eb7b22d1d2e13b172
1 file changed