Refactor PTransformOverrideFactories to remove code duplication

The majority of the factories take in a single PCollection as input and produce a single PCollection as output; the code to extract the single input can be invisible as part of the class hierarchy, rather than explicitly called in every override.

Imported from Jira [BEAM-1982](https://issues.apache.org/jira/browse/BEAM-1982). Original Jira may contain additional context.
Reported by: tgroh.

Start by locating PTransformOverrideFactories and reading the related class hierarchy and factory overrides. Check how each factory extracts its single PCollection input; done when that common extraction is handled by the hierarchy and the duplicated calls are removed without changing factory behavior.

Written by the indexing model from the issue text.