Add nullability support for CarrierConfigLoader class am: fe0d7c1cb6 am: 6f007e2396

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/services/Telephony/+/17764636

Change-Id: Ia89d45ab6427d96b01a938808fd20ba1c55b3c3a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>