Allow changing rotary radius values via overlay

This change move the rotary radius settings from constants to
config values that can be overridden by a device.  This is
required to support the larger radius background images
for tablet devices like the nook color.

This change also changes the hdpi assumed dpi settings from 160 to
180.  This will allow for the 161 dpi hack for the NookColor and
other tablets.

Change-Id: I47f8de0923d10833479e44228a9a614f11723d78