Fixing non-delivering of DRAG_START event to each view

The problem was that GroupView.dispatchDragEvent didn’t send ACTION_DRAG_STARTED
to GroupView itself in case when there is a child that is interested
in this drag.

Now, delivering DRAG_STARTED to all children and to GroupView itself.

Also corrected delivery of DRAG_ENDED to match docs: it will be delivered
only to the views that returned true from DRAG_STARTED.

Same story with Enter, Exit and Location notifications: they are delivered
only to children that returned true from DRAG_START.

Delivery to
the ViewGroup itself never depends on a child’s return value.

Bug: 22028725
Change-Id: I44495d6da8423c9cc012f029233f4604cb60c74b