Reference to ParallelBundleManager attr "_skip_registration" should be "_registered"

It appears there was a mistake in the multiprocess refactor of the BundleManager. I came across this issue while getting mypy type analysis setup (score one for static type checking!)

The offending line is here: https://github.com/apache/beam/blob/8d2997f8d7ad84649b8ecb2f7e2ca2eceb91b6d0/sdks/python/apache_beam/runners/portability/fn_api_runner.py#L933

I think it should be using "_registered", as seen here: https://github.com/apache/beam/blob/8d2997f8d7ad84649b8ecb2f7e2ca2eceb91b6d0/sdks/python/apache_beam/runners/portability/fn_api_runner.py#L1909

I can easily change the attribute name, but presumably this will change the behavior. Hopefully for the better, but I don't know enough to say.

Imported from Jira [BEAM-8486](https://issues.apache.org/jira/browse/BEAM-8486). Original Jira may contain additional context.
Reported by: chadrik.