Refactor: process mentions of class object

Rename usages that refer to "default object" concept now
Test data file names are left as is

core/serialization.jvm/src/jvm_descriptors.proto

@@ -52,7 +52,7 @@ message JvmFieldSignature {
   required int32 name = 1;
   required JvmType type = 2;
 
-  // True iff this field is a backing field for a class object and is really present as a static
+  // True iff this field is a backing field for a default object and is really present as a static
   // field in the outer class, not as an instance field here
   optional bool is_static_in_outer = 3 [default = false];
 }