Thread-consistent snapshots

Bug: 188814480

Once a thread starts using a snapshot, it will continue using the same
snapshot even if the call stack exits and reenters PM.

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: Id7e04e603147020c37c900ab69994c10211a9048