Refine generic signature for Map.get/remove

Before this change generic signature wasn't written because of wrong
assumption about it absence in all cases where we replace generic parameter
with Object

compiler/testData/codegen/boxWithJava/collections/readOnlyMap/J.java

@@ -0,0 +1,13 @@
+import java.util.*;
+
+public class J {
+
+    private static class MyMap<K, V> extends KMap<K, V> {}
+
+    public static String foo() {
+        Map<String, Integer> collection = new MyMap<String, Integer>();
+        if (!collection.containsKey("ABCDE")) return "fail 1";
+        if (!collection.containsValue(1)) return "fail 2";
+        return "OK";
+    }
+}