Add support for autoloading callback extension.

This works by checking for the property vulkan.enableLayerCallback. If the
property doesn't exist, is false, or the app has explicitly loaded the callback
extension the change does nothing. If the property is enabled and the app
has not loaded the extension this change forces the loader to enable it.

Change-Id: I52f7afb4136358a360aaf62188d98b34b563778a
(cherry picked from commit adeac8e46bd0034321ea348132d3f9db967b72e9)