Implement display overscan support.

The window manager now keeps track of the overscan of
each display, with an API to set it.  The overscan impacts
how it positions windows in the display.  There is a new set
of APIs for windows to say they would like to go into the
overscan region.  There is a call into the window manager to
set the overscan region for a display, and it now has a
concept of display settings that it stores presistently.

Also added a new "wm" command, moving the window manager
specific commands from the "am" command to there and adding
a new now to set the overscan region.

Change-Id: Id2c8092db64fd0a982274fedac7658d82f30f9ff