Disallow drag and drop of an activity from triggering autoenter PIP

- The user is explicitly replacing an existing app with the dropped app
  and does not expect the leaving app to break the split pair and enter
  PIP

Fixes: 312633296
Test: Split with an auto-enter PIP app, and drag to replace that app
Change-Id: I44e3a1dad364e9ce54dbcecc2dc7b03473cc0f7b