[jnigen] Interface implementation (https://github.com/dart-lang/jnigen/issues/326)

* attach finalizable to close the port for interfaces 
* add missing JCharacter boxed type
* add .implement method completed for interfaces
* fix descriptor issues
* temporarily do not check the ffigen bindings until ffigenhttps://github.com/dart-lang/jnigen/issues/555 is solved
* remove duplicated java source in jni + sort ffigen.yaml inline functions
* add the interface implementation under an experiment flag + docs