Log app launches via taskbar drag and drop

- This also goes through the app predictions pipeline to update
  hotseat predictions

Fixes: 316940471
Test: Drag apps to split screen, ensure taskbar predictions update
      (also verify by logging AppPredictor#notifyAppTargetEvent)
Flag: None
Change-Id: I844414ecb56b4e36a2c94d913c1fb37abf056593