PackageManager lock reduction: Settings on-change

Bug: 161323622

Improve change detection in the Settings class.  Attributes used in
the snapshot are now watched.

Test: atest
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests
 * FrameworksServicesTests:WatcherTest

Change-Id: Icc4a81900d2014e09cabbb2628702c6c20fc4da6