Merge "Expand documentation for PURPOSE_AGREE_KEY" am: 6d3bd2dca6 am: 4f8bcd4261

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1587048

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I8902f84a28b5e588f45e9150aa5c7dc51db73174