Cherry-pick: ICU-21875 improve C API coverage am: 2919e5e2c8

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/icu/+/17918903

Change-Id: Ieaf65d58f7db733a3debf264579165c1ddd7ce7a
Ignore-AOSP-First: this is an automerge
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>