[xtext][encoding] Make backwards binding of encoding provider backwards compatible

Also mark the stateful instance as a singleton.

Change-Id: Ic4e20432a228b5c94b58c81f1888feed65932ab0
[purexbase][tests] Fixed test which seemed to fail because of waitForAutoBuild()

Change-Id: I03844b1a11da7f1a57d9324ac0be4879445568f9