Add HFP AIDL design am: e50be86fd4

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

Change-Id: I49d95135d541aaae443ab57ff9271c1752e72557
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>