Extract common class for WithTimestamps.AddTimestampsDoFn and Create.TimestampedValues.ConvertTimestamps

It seems like these APIs are predominantly duplicative of each other and, that it's hard to find one of them if you knew about the other.

https://github.com/apache/beam/blob/348d335883b14a9b143b65e4b3c62dc79f62d77e/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/WithTimestamps.java#L134

https://github.com/apache/beam/blob/348d335883b14a9b143b65e4b3c62dc79f62d77e/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Create.java#L560

What would make the most sense to me is if TimestampedValues were implemented in terms of both Values and WithTimestamps. I'm still learning about Beam though -- would this approach cause some kind of performance problem?

Imported from Jira BEAM-1811. Original Jira may contain additional context. Reported by: wtanaka.