Clear all DragView in onResume

- Multiple users reports view getting stuck on screen. This CL
  clears all DragView when user leaves launcher.
- Moved to onResume to not interfere with start up time,
  and also aligned with when we remove AbstractFloatingView.
- Root cause still undetermined, but happens enough that it warrants
  a temp fix.

Bug: 162636845
Test: drag and drop configurable widget
      (widget that gets dropped then opens up a
       configuration activity)
Change-Id: I532b5274a1308c53f67c23c5c8ca07f2b7f9802c