Merge "Support key without default value for carrier config subset retrieval APIs" am: f303bd31e9

Original change: https://android-review.googlesource.com/c/platform/packages/services/Telephony/+/2479118

Change-Id: I0f3218a407dde7a2dc7d1ebcc33054d7beda1d14
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>