This commit makes two changes to fix jruby#6196:

* When both an inner class and a static final field have the same
  name, only define the field. Typically this is used for a
  singleton pattern where the field points at a singleton instance
  of the inner class, as with Kotlin companion classes. Since we
  can't define both, we prefer the field.
* Do not define the inner class constant as a side effect of
  initializing that class's proxy, like we do for package
  constants for normal classes. Defer that constant definition to
  the initialization of the declaring class, which will make the
  decision whether to define (or not) the constant at that point.

If both the field and the class are needed, it's still possible to
import the inner class under a specific name using `java_import`:

```ruby
java_import("OuterClass$InnerClass") { "MyWeirdInnerClass" }
MyWeirdInnerClass.whatever()
```