Add nullability support for CarrierConfigLoader class am: fe0d7c1cb6 am: e1e6d57739

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

Change-Id: Id1fda46a006a44d2b2a821210c7cde5f36b04afc
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>