Use stack bounds to determine input bounds for window.

We were previously using the task bounds to determine the input
bounds for the window. This doesn't work for the case where the
docked stack is next to a non-resizeable activity/task. In this
case the task is still fullscreen, but the window surface is
cropped to the stack size which isn't fullscreen since the docked
stack is up. By using the stack bounds the input region matches
what is displayed on screen.

Change-Id: Ia4d2b3a7a050eff38d651e511f5822c4428e137d