Add nullability annotation with best judgment

[chao2zhang]

I enabled UnknownNulless lint check to detect violations, and use my best judgment to fix the nullability for Kotlin interoperability.

• If nullable causes crash, fix the code and annotation with nonnull.
• If all internal callers are expecting nonnull, annotation with nonnull.
• If any internal caller passes nullable, annotation with nullable.
• If nullability is missing from its parent class, leave as is.