Refactor of BandController/DragStarter/GestureDetector.

- DragStartHelper refactored into DragStartListener, and has no
  dependency on framework class
- BandController setup code with RecyclerView moved to
  ListeningGestureDetector
- Bulk of code to start drag (drag shadow, Clipper data, drag
  title/icon) moved to DragStartListener
- Remove scrolling ability with gesture if using mouse

Change-Id: I8adf7edee6adcf4db3354df757a95e601397066e