Re-enable Nimbus with Off the main thread I/O and non-blocking networking

[jhugman]

Fixes SDK-152.

In an effort to fix SDK-157 this PR brings together:

1. SDK-170 Add a Threadsafe annotation for interfaces mozilla/uniffi-rs#372 — allow Rust objects to be marked as Threadsafe, to indicate to uniffi not to lock access to one method at a time.
2. Make NimbusClient Send+Sync via mutexes and cells mozilla/nimbus-sdk#86 — refactor Nimbus to use its own locking, instead of relying on uniffi. This also introduces an in-memory cache of enrollments.
3. Split update_experiments into two phases mozilla/nimbus-sdk#78 — split the long running, blocking and single threaded updateExperiments into a slow network fetch and a fast apply phase to re-calculate enrollments which writes to the database.
4. Add methods to cover new methods in NimbusClient android-components#9394 — Move fetch calls and calls that mutate the internal state of Nimbus to two separate coroutine scopes.

Furthermore, this PR also: moves setup of the Nimbus object into its own file; starts using the new fetch and apply methods, and improves error resilience.

Once all of this lands, this fixes #17086. Indirectly, it fixes #17143.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not Has no UI
• [ ] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product. Has no UI.

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[jhugman]

IMPORTANT: Only turn this back on once the following issues are resolved:

• [Bug] Large regression in MAIN/VIEW start up due to Nimbus network IO #17086: Calls to getExperimentBranch seem to block on updateExperiments causing a large performance regression loading the home screen.
• [Bug] Crashes initializing Nimbus #17143: Despite having wrapped getExperimentBranch/withExperiments in a catch-allusers are still experiencing crashes.

#17143 is fixed by way of SDK-161 and mozilla/nimbus-sdk#77 and extra try/catch protection around Nimbus setup.

[travis79]

🎉

[travis79]

nit: blank line

[mergify]

This pull request has conflicts when rebasing. Could you fix it @jhugman? 🙏

[csadilek]

We did a few rounds of manual testing, simulating slow networks and no network connection at all. Everything seems fine now :).

At this point we can land and should monitor Sentry and perf. results.