Validate if pointer is valid in ViewDragHelper.

Some users of ViewDragHelper do not send all the events to it and
it is possible to get to a state where ViewDragHelper receives an
ACTION_MOVE without ACTION_DOWN beforehand. This CL makes it ignore
such events and warns the developers about this issue.

Bug: 27199384
Change-Id: I13fbc6479c68042803d8d6b78dd7cdab700d44be
1 file changed