Make PM unit tests work with trunk stable flags

Currently DeviceConfig is fully mocked out, so annotations like
@RequiresFlagsEnabled don't work. This CL makes DeviceConfig mock
fall back to the real instance.
One side effect of it is that a flag, SYSTEM_EXEMPT_FROM_SUSPENSION,
that defaulted to `true` is now also considered `true` in tests,
causing a failure, so I also cleaned up the flag and updated the
test code to provide AppOpsManager mock.

Bug: 309671525
Bug: 263464464
Test: atest com.android.server.pm.PackageManagerServiceBootTest \
      com.android.server.pm.PackageManagerServiceHibernationTests \
      com.android.server.pm.ApexManagerTest \
      com.android.server.pm.DeletePackageHelperTest \
      com.android.server.pm.PackageFreezerTest \
      com.android.server.pm.PackageArchiverTest \
      com.android.server.pm.PackageMonitorCallbackHelperTest \
      com.android.server.pm.InitAppsHelperTest \
      com.android.server.pm.SharedLibrariesImplTest \
      com.android.server.pm.SuspendPackageHelperTest \
      com.android.server.pm.DistractingPackageHelperTest
Change-Id: I17bdfc498d86c43faf257ef64b6b0d20069e5cc8