Change mouse pointer when drag and drop is active

Mouse pointer is set to STYLE_GRAB when the drag has started and
reset to STYLE_DEFAULT when the drag has ended.

Resetting the pointer shape to the one defined by an underlying
view will be handled in a separate patch.

Bug: 24415739
Change-Id: I8df0a08c5701a34a48f10ec6b43c2cf2e6362d61