PackageManager lock reduction: Settings

Bug: 161323622

Modify the Settings class used by PackageManagerServer to be
Watchable and Snappable.

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

Change-Id: Ibf6b2b5fb04b39489e52d6a25cce622bce32a805