[jnigen] Fix summarizer nested class bug (https://github.com/dart-lang/jnigen/issues/312)

* Fix inner classes
* Fix nested class finding
* Remove -r and -v options
* Add checks for nested classes in config
* Remove problematic steps from GH action
* Add anonymous inner class example
* Use sorted() for deterministic order
* Refactor ClassFinder logic and add Unit test
* Filter anonymous classes in summarizer itself