Add nullability support for CarrierConfigLoader class am: fe0d7c1cb6

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

Change-Id: I2e16678e4af9c557ebf6036e86287fcb4a1bcee9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>