Allow split with an existing split task

- Determine the task in the group being clicked in split-select mode
  and launch split using that selected task
- Also make sure we don't handle drag cancel twice when we are animating
  the return of the drag surface

Bug: 219060441
Test: Split from home/overview with fullscreen+fullscreen task, and with
      fullscreen+split task

Change-Id: I48ec0a82812197803ff4b3698830a9cb705719e3