Add HFP AIDL design am: e50be86fd4

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/2774933

Change-Id: I77bd9347d6b226b5320a021e239d2f63d7232a0b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>