Merge "Expand documentation for PURPOSE_AGREE_KEY" am: 6d3bd2dca6 am: 4f8bcd4261 am: a06ffa1395

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I04bc22715f7908c820a1ac4db2ea65352042acde