Constrain 'touch modal' behavior to the activity stack.

Currently a 'touch modal' window grabs drag events
from the entire screen. Since 'touch modal' is the
default, this makes dragging between apps impossible.

This change allows such grabbing behavior only for
events that are still over the same activity stack.

Bug:19548858
Change-Id: I7d48b58e7fcb620521361e17cb70914913afae03