Code to enable dragging to System UI shelf.

This CL doesn’t let the new code run, it’s under a compile-time flag,
and the behavior should be same as before.

The change introduces a concept of a DragDriver, which encapsulates
behavior of the existing DND (InternalDragDriver) or the framework-
driven DND (SystemDragDriver).

An instance of DragDriver gets created by DragController for the time
while a DND operation is in progress, and it takes care of translating
DND events for DragController.

Also did some cleanups, like removing meaningless fields etc.

Plans for future: keep working in Burnaby, and:
* Perhaps, better separate DragDriver from DragController
* Detect fling gesture for system DND
* Look at accessibility
* Polish animations and appearance
* Fix dragging from folders
* Support cancelling DND

Then, it will become possible to enable new dragging (based on Android
version).

Bug: 22028725
Change-Id: I41b40e9d512d83937f5b101ac8e3e8e7e807c269