Add typedefs and nullness annotations.

This changeset adds in typedef annotations (custom annotations
marked with @IntDef) for various int parameters and return values
in the API. It also adds nullness annotations for cases where the
documentation explicitly mentioned null policy, or where it was
blindingly obvious from the context. Also fixed some typos in the
documentation.

Change-Id: Ica27c01368895818e26237544edd8483007155bb