Refactor PTransformOverrideFactories to remove code duplication

[kennknowles]

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. Original Jira may contain additional context.
Reported by: tgroh.