Use PrintableString in oatdump.

And enhance PrintableString to assume modified UTF-8, which is all we ever give
it. \u0000 is more readable than \xc0\x80 to most people.

Change-Id: I45bd8d65694eda0ef4ef03abc40f41a76f07a671
diff --git a/src/oatdump.cc b/src/oatdump.cc
index d2a78af..af084d5 100644
--- a/src/oatdump.cc
+++ b/src/oatdump.cc
@@ -840,8 +840,8 @@
       StringAppendF(&summary, "%p: java.lang.reflect.Method %s\n", obj,
                     PrettyMethod(obj->AsMethod()).c_str());
     } else if (obj_class->IsStringClass()) {
-      StringAppendF(&summary, "%p: java.lang.String \"%s\"\n", obj,
-                    obj->AsString()->ToModifiedUtf8().c_str());
+      StringAppendF(&summary, "%p: java.lang.String %s\n", obj,
+                    PrintableString(obj->AsString()->ToModifiedUtf8()).c_str());
     } else {
       StringAppendF(&summary, "%p: %s\n", obj, PrettyDescriptor(obj_class).c_str());
     }