Add Configuration data for round displays

Add round values to the screenLayout field for Configuration
and a convenience method to check roundness.

Plumb this through the DisplayManager, making roundness the property
of a DisplayAdapter. The built-in main display will read the
configuration resource config_mainBuiltInDisplayIsRound to determine
its roundness. Device-specific resource overlays should set this to
true for devices with round primary displays.

By default, this config resource inherits from the existing
config_windowIsRound value currently used by some Android Wear
device configurations.

This change awaits another for aapt/native resources code to make the
resource filtering system aware of this property.

Change-Id: I1daced7ca6d6e172789e7c32bebfd43753bfa2ae