In jruby#8146 we saw that DelegateClass can be broken by
JRuby extensions that do not explicitly set their initialize
methods to Visibility.PRIVATE. This is an error-prone requirement
as detailed in jruby#8210, since there's no indication that
initialize is special and should always be private.

This patch introduces a "DEFAULT" visibility to be the default for
the annotation. When binding methods using annotation processing,
the DEFAULT visibility will translate itself to PRIVATE for the
initialize method and PUBLIC for other cases. This ensures that
initialize is always set to PRIVATE if no other visibility is
specified, avoiding the bug. If initialize is set to some other
visibility, it will keep that visibility.

Fixes jruby#8210