ci: Check for unexpected Room schema diffs

[nikclayton]

At the moment CI can succeed, but the app can crash on startup if there was an unexpected change to a room schema in a PR.

To detect this add a new CI check.

Should probably look for two things:

1. A PR that changes an existing schema JSON file. AIUI that shouldn't happen, any schema changes should be accompanied with a version bump and a new schema file.
2. A PR that doesn't change the schema, but the schema is changed after the app is built. This indicates a change in one of the DAO files. Building the app then running "git diff" in the schema directories and confirming there are no diffs is probably sufficient (as any schema changes should be included as part of the PR).