Uses PMI.PackageListObserver in AMS to listen to new packages.

PackageMonitor is BroadcastReceiver-based and occasionally takes a while
(30+seconds) before being notified about new packages, especially during
high-activity moments like CTS.

PMI.PackageListObserver is a way for system_server services to
directly listen to package changes without relying on broadcasts and does not suffer the same delays.

Bug: 261217661
Test: go/abtd to check flakiness in CtsAccessibilityServiceTestCases
Change-Id: I8b77c25de2f7f128f921f83ace9013e53dee1a6e