Don't set input bounds for task to the stack bounds.

3b265801 tried to fix a
problem where the task could be dragged outside its stack
bounds. However, the input bounds was been set to the
entire stack bounds which prevented input from going to
other tasks in the stack.
We now intersect with the stack bounds.

Bug: 24201913
Change-Id: Ibb84bc099b6709ceb865f114b5e9857c5ab2ef1a