Auto-generate docs based on annotations.

We have a handful of annotations that we've been sprinkling across
the platform APIs, such as @Nullable, @NonNull, @IntDef, etc.  It
would be really helpful to surface these contracts to developers
through the SDK docs.

This change allows annotations like those mentioned above to declare
the following new javadoc fields:

@memberDoc: docs to append to a field or method definition.
@paramDoc: docs to append to a @param definition.
@returnDoc: docs to append to a @return definition.

This change also builds a docstring to describe the list of all
constants listed in an @IntDef annotation.  Sadly AnnotationDesc
only passes along raw constant values, so we need the help of the
new "prefix" annotation argument to help find the field names.

Test: builds
Bug: 37526420
Change-Id: I4cfc00dd904e5dfa945b406d546e36846b7c0c28