Add nullability support for CarrierConfigLoader class am: fe0d7c1cb6 am: 6f007e2396 am: d0aee12a77

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

Change-Id: I48e16b9d0c767e81b2d3239e1bd7e36a237c5d59
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>