PubSubIO.readAvros should produce a schema'd PCollection if clazz has a schema

[damccorm]

Currently PubsubIO.readAvros(clazz) always yields a PCollection with an AvroCoder. This should only be a fallback in the event that no coder can be inferred. That way if we can infer a schema for clazz we will produce a PCollection with a schema.

Imported from Jira BEAM-8330. Original Jira may contain additional context.
Reported by: bhulette.