Implement View.updateDragShadow

The new method allows changing the drag shadow image
while the drag is in process.

It has to be called on a View in the same window (under the
same ViewRootImpl) that the view which started the drag.

Bug: 25520420
Change-Id: I57e752626a4a6a86d42c597c09dbd7ec77f42f30
4 files changed