Ensure that the heartbeat background task is stopped before we close
the users as it accesses data within these instances.
Additionally, we also make sure that when telemetry is disabled, we stop
the background task.
Finally, `HeartbeatManager` now specifies what the desired interval is
so we can better configure the test cases.
When performing a factory reset, we don't want to wipe all keychain
entries. The only keychain entry should be the vault's passphrase,
and we need this to be able to decrypt the vault at next startup
(to avoid it being reported as corrupt).
Update UIDValidity to be timestamp with the number of seconds since
the 1st of February 2023. This avoids the problem where we lose the
last UIDValidity value due to the vault being missing/corrupted/deleted.
