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

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

Change-Id: I76d9768ac5376c11d3a295f32e846aa97b484a67