Enable PackageManagerService snapshots

Bug: 161323622

Throw the switch that enables PackageManagerService snapshots.  Revert
this CL if the snapshots introduce field failures.

Test: atest
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser

Change-Id: I1a35ae759a4f90cc1c378f0f384c331fe3d27e0d