Fixes testSplitWindow cann't be retried

MultiWindowEmptyActivity stay on screen and does not be cleaned cause
the next cycle can not launch new activity. Adds cleaner function to
remove the activity.

Bug: 177029546
Test: atest CtsAutoFillServiceTestCases:MultiWindowLoginActivityTest#testSplitWindow
Change-Id: I9587228e0e5cbcc9e707b5f09dc3441b0843a673