Initial draft of high-level multi-display APIs.

This patch introduces the ability to create a Context that
is bound to a Display.  The context gets its configuration and
metrics from that display and is able to provide a WindowManager
that is bound to the display.

To make it easier to use, we also add a new kind of Dialog
called a Presentation.  Presentation takes care of setting
up the context as needed and watches for significant changes
in the display configuration.  If the display is removed,
then the presentation simply dismisses itself.

Change-Id: Idc54b4ec84b1ff91505cfb78910cf8cd09696d7d