Ensure PM watches all Watchables

Bug: 179388643

The bug occurs because two Watchables were not being observed by
PackageManagerService.  The two Watchables are mSettings and
mInstantAppRegistry.  The bug should have been detected by
Watchable.verifyWatchedAttributes() but that, too had a bug: the
PackageManagerService attributes were not visible to the Watchable
package so the verification code got an IllegalAccessException, which
caused the attributes to be silently ignored.

The following changes are made:
 1. PackageManagerService now registers with the missing Watchables.
 2. Registration is moved to a function that is called in two places.
 3. The verification code is enhanced to ensure fields are
    accessible.  A RuntimeException is thrown if the field is still
    not accessible.  Note that only fields annotated with @Watched can
    generate this exception.
 4. PackageManagerVerification is skipped (log-only on errors) if it
    appears to be part of a mockito test.
 4. Settings now registers with a missing Watchable.  The Watchable is
    not actually used in Settings (see the TODO at the attribute
    declaration) but this CL assumes that it might be used in the
    future.
 6. One import order violation was corrected.

In addition to automated tests, the changes were tested with an
instrumented PackageManagerService that enabled snapshots and skipped
registration with one or more observers.  All attributes were tested
one by one.

Test: atest
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser
 * android.appsecurity.cts.EphemeralTest#testEphemeralStartExposed06
 * android.appsecurity.cts.InstantAppUserTest#testStartExposed06
 * com.android.server.pm.PackageManagerServiceBootTest

Change-Id: Ib2b14c4745bd5e5ab2882ed6fe953d7da2df4087