Merge "Add some comments about thread-safety" am: d0c6e0fa am: 8b1745b2 am: 964778fe

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1503391

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: If64310c84837082a2172a4d9b30c2068370bd6fb