Trigger AST should use @AutoValue

Triggers are already enriched POJOs that could use `@AutoValue`. Certainly once they are converted to a simple AST this is the way to go.

Imported from Jira [BEAM-766](https://issues.apache.org/jira/browse/BEAM-766). Original Jira may contain additional context.
Reported by: kenn.

The issue names no files, tests, or entry points. Start by locating the Trigger AST and its current enriched POJO definitions in the Java source, then trace their construction and existing coverage. Done means the appropriate Trigger AST classes use @AutoValue without breaking the relevant Beam tests.

Written by the indexing model from the issue text.