Improve snapshot performance

Bug: 185481065

Use caches to improve snapshot performance.  SnapshotCache.Auto is a
concrete SnapshotCache that can be applied to a Snappable.  All the
Watched* classes are Snappable.  The cache improves performance and
reduces memory churn.

Test: atest
 * CtsContentTestCases:IntentFilterTest
 * CtsDynamicMimeHostTestCases
 * CtsRoleTestCases
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser
 * FrameworksServicesTests:WatcherTest
 * android.appsecurity.cts.EphemeralTest
 * android.appsecurity.cts.InstantAppUserTest

Change-Id: Ia993613f566cb86b145ddf1d5280e8780252a951