Generate @IntDef annotations for nanoproto enums.

@IntDef is a support library annotation which allows build tools to
determine the valid set of values for a given integer field when that
field is intended to be restricted like an enum. This avoids the
overhead of enums while still allowing for compile-time type checking
in most circumstances.

Change-Id: Iee02e0b49a8e069f6456572f538e0a0d301fdfd5