Add nullability support for CarrierConfigLoader class am: fe0d7c1cb6

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

Change-Id: I16773f519d3295d5855ec4843a00a03cff4d2d8e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>