Refactor pkg/proxy/config's ServiceConfigHandler and EndpointsConfigHandler to have different update methods.

Refactor `pkg/proxy/config`’s ServiceConfigHandler.OnUpdate and
EndpointsConfigHandler.OnUpdate to different method names as they have
different signatures.

This will let the new proxy
(https://github.com/GoogleCloudPlatform/kubernetes/issues/3760)
implement both interfaces.

Since we won’t need a separate loadbalancer structure (load balancing
is handled in the proxy rules), we will simply handle both event types
from the same object.