Make input system aware of multiple displays.

The input system needs to know about the window that has
focus, even if it is on a secondary display.  So now we
send it the list of all windows and indicate which display
they are on.  We filter the list of windows as necessary
when delivering touch events.

To keep things simple, monitor input channels and input
filters are not supported except on the main display.
We also do not pass the display id to applications; it is
only used inside the input system for now.

Properly scale touch coordinates based on the viewport.
This will be needed to ensure that touch works on external
display as well as when the internal display is being used
to simulate a different resolution.

Change-Id: I1815579a52fcc852c519b5391fc7ab8767c2dc59