Annotate type wrapper objects' getValue() as @NonNull

Wrapper objects are enforcing the hard-coded property ('value') to be non-null, but this is not reflected in the getter nor do we take advantage of non-nullness during hashCode()/equals() implementations.

Add the ability for TypeGenerator to mark individual (simple?) properties as non-nullable and take it into account in the java-api-generator, propagating it to fields and getters.