FIR Java types: add Kotlin/Java mapping & mutability enhancements

Related to KT-29937

compiler/testData/loadJava/compiledJava/kotlinSignature/propagation/return/SubclassOfCollection.fir.txt

@@ -1,4 +1,4 @@
 <E> public abstract interface SubclassOfCollection : R|java/util/Collection<E>| {
-    public abstract operator function iterator(): R|ft<java/util/Iterator<ft<E, E?>>, java/util/Iterator<ft<E, E?>>>|
+    public abstract operator function iterator(): R|ft<kotlin/collections/MutableIterator<ft<E, E?>>, kotlin/collections/Iterator<ft<E, E?>>>|
 
 }