[TextClassificationManagerService] hold PackageMonitor object

System server only holds a weak reference to PackageMonitor though PackageMonitorCallback. If there's nothing else holding on to PackageMonitor, it will get GC'd and following callbacks to it will be discarded.

BUG: 398759145  
TEST: manual
FLAG: EXEMPT bug fix
Change-Id: Id144aeac1d9f41ea6a4d9f36af769c0bdd783ebc