This patch introduces a new "FennecSettingsMigrator" which will be responsible
for migrating all common Fennec - Fenix settings.

Starting with the telemetry opt-in status which in Fenix will be "true" if the
Firefox Health Report setting ws also true on Fennec, and "false" otherwise.
It may be possible that the Fenix install is a new one and not an upgrade from
Fennec in which case no setting will be migrated and the migration would be
considered as successful.

Co-authored-by: Grisha Kruglov <gkruglov@mozilla.com>