Fix the PMS live computer

Bug: 180050592

The live computer was created before PackageManagerService
mInstantAppResolverConnection was initialized.  Unfortunately, the
live computer is used to initialized mInstantAppResolverConnection.
This change rebuilds the live computer after
mInstantAppResolverConnection is finally initialized.

Two additional cleanups are included, related to the use of the live
computer.
 1. The ComputerEngineLocked now extends ComputerEngineLive.  The
    Locked class inherits the overriedes from the Live class, and the
    now-redundant overrides in the Locked class are removed.
 2. mSnapshotComputer is initialized to null in the constructor for
    PackageManager Service.  The snapshot should not be used until the
    end of the constructor; setting the attribute to null will trigger
    an NPE if the snapshot is used improperly.

See the bug description for specific tests.  Instant apps are now
launched correctly.

All atests pass 100%.

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: Ic7e5a3ae9d661e2a7079d40a0f1a1ca6dc5badf2