Add canChildBeDraggable() method to SwipeHelper's callback

This callback checks the given child is actually draggable or not.
This method is need for ARC, since a child may not be draggable in
some cases.

FYI: ag/3244118 is the CL for NYC ARC.

Bug: 63874929
Bug: 62602530
Test: Compile and ran 'runtest systemui'
Change-Id: I085f1c1c72240529a19e0dcb1f543e7cb36899f0