fix(GODT-3125): Heartbeat crash on exit

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.

internal/app/bridge.go

@@ -113,6 +113,7 @@ func withBridge(
 		crashHandler,
 		reporter,
 		imap.DefaultEpochUIDValidityGenerator(),
+		nil,
 
 		// The logging stuff.
 		c.String(flagLogIMAP) == "client" || c.String(flagLogIMAP) == "all",