Fix mock mode

[ZacSweers]

Currently, mock mode doesn't do anything. I'd like to evolve this in a few ways

• make it functional again for totally offline work with stub data. Unclear if this means dummy JSON data at the request level or something more like retrofitmock. Latter is more practical, but former hooks in nicely with the next bullet. Maybe we can do both.
• I think it would be really neat if recent responses for certain services could somehow be cached and included in a bugsnag reports. This would make debugging issues or crashes much easier.
• services need a debug-only API hook to potentially provide their data. I'm not sure how to do this in a sane way across Gradle module boundaries.

[ZacSweers]

Elaborating on the second point: if a private bug reporting mechanism was instituted in such a way that a logging interceptor was enabled for the scope of a report and sent up privately (such as to Bugsnag), that could be possible. Can't do it automatically without hurting perf, but feasible. Some services might be able to support this via unique page IDs (reddit)