Correct snapshot races

Bug: 185481065

This fixes the race conditions that could theoretically cause
PackageManager snapshots to fail, especially after package
installation.  No failures have actually been observed.

1. Three arrays in Settings are now watched.  Other attributes that
   are included in the snapshot are annotated @Watched(manual = true).
   untrackedStorage() is used for these arrays during XML read/rwrite
   operations.

   Also, KeySetManagerService now takes a WatchedArrayMap from
   Settings, instead of untrackedStorage().  KeySetManagerService
   should not alter the array, but if it does, Settings will learn
   about it.

2. Key attributes in SnapshotCache are volatile.  This should not be
   necessary, since snapshots are only modified under PM mLock, but
   the change is made just in case.

3. PackageManagerService.sSnapshotInvalid is now an atomic.  This
   should eliminate narrow race conditions around the variable.

4. A corking facility is part of PackageManagerService.  Corking is
   not currently used but it could be valuable to suppress snapshot
   creation during very busy periods.  This is the same reason for
   corking that PropertyInvalidatedCache uses.  SnapshotStatistics can
   track corking.

5. SnapshotStatistics has a "brief" mode that can be invoked from the
   command line.  Brief output provides a good overview of snapshot
   behavior.  Full output (which is the default) is good for more
   detailed analysis.

Test: atest
 * CtsRoleTestCases
 * CtsContentTestCases:IntentFilterTest
 * CtsDynamicMimeHostTestCases
 * 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: I111cc4256cbd795fc2aba287f57cfcfd5f9cd7ab