Support for devices with a chin.

Information about the chin is now part of the config.xml instead of the
theme. It is retrieved by WindowManagerService and passed to the clients
as insets. Clients can adjust their behavior in a way that makes it
invisible to the user, that part of the surface doesn't actually exist.

Bug: 19908853

Change-Id: Iedf57bf3c848201b854f91ffeb3b59187d375c1f