Merge "Update docs and add nullability annotations" am: 9c0f722e2e am: c7898c9355 am: 3f75fdeb9d am: f8cb55dfd1

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1455141

Change-Id: I4f401cc7d5ae71a95ccd47fe2f42a939875b7dcb