Disable snapshots for mocked tests

Bug: 180107750

Disable snapshots inside the PackageManagerService test constructor.
Enable snapshots inside the PackageManagerService "operational"
constructor.  The global constant SNAPSHOT_ENABLE is still used.

When snapshots are disabled, do not register any observers.

Test: atest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:ScanTests
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * PackageManagerServiceBootTest
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser
 * android.appsecurity.cts.EphemeralTest
 * android.appsecurity.cts.InstantAppUserTest

Change-Id: I4eb66ffd4d59a7981868691c58b4b237cb6b206f