Add display manager skeleton.

The purpose of this change is to remove direct reliance on
SurfaceFlinger for describing the size and characteristics of
displays.

This patch also starts to make a distinction between logical displays
and physical display devices.  Currently, the window manager owns
the concept of a logical display whereas the new display
manager owns the concept of a physical display device.

Change-Id: I7e0761f83f033be6c06fd1041280c21500bcabc0