Make mirroring automatic based on Windows on display.

Tell the display manager whenever a given logical display
contains interesting windows.  If so, then the display
manager arranges to show that content on a physical display,
otherwise it ignores the logical display and makes its
associated primary physical display mirror the default
display.

Assign DisplayContents when Displays are added, remove them when
Displays are removed, and update the DisplayInfo when Displays
change.

Change-Id: I36e08ec538055acabe1e24cdd12c40de4e47a158