Start moving away from DisplayMetrics.DENSITY_DEVICE.

This puts in most of the infrastructure needed to allow us to
switch between different densities at run time.  The main remaining
uses of the global are to initialize the Bitmap object (not sure
what to do about that since it doesn't have anything passed in
the constructor to get this information from), and being able to
load drawables if we need a different density than what was preloaded
by zygote.

Change-Id: Ifdbfd6b7a5c59e6aa22e63b95b78d96af3d96848