Merge "Add KM_PURPOSE_AGREE_KEY constant." am: 203813672e am: 4f136a5659

Original change: https://android-review.googlesource.com/c/platform/hardware/libhardware/+/1556301

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I6a7edf8d299a04dd864b6fc2f7a6a8505d00d39f