Fixes https://github.com/mozilla-mobile/fenix/issues/3211

There were two issues with the previous implementation:
* We can't write to the cache concurrently and in this case edit() will
  return null. We should synchronize our writes to avoid not writing
  anything to disk while another write is in-flight.
* In cases where edit() still returns null (stale snapshot) we just
  bail out.

- Refactor `ShippedDomainsProvider` and `CustomDomainsProvider`. Using
composition instead of inheritance.
- Add tests for `BaseDomainAutocompleteProvider`.