Stops using the deprecated PackageListObserver in A11yManagerService.

This was originally added because PackageMonitor was slow. That slowness
was resolved so PackageListObserver is now unnecessary.

There should be no observed change in behavior, but feature flagging
this as required to be safe.

Bug: 304561459
Test: atest CtsAccessibilityServiceTestCases
Change-Id: Ief37031d81b05817e285cd5cff0c8e8513c6a602