Rename String.concat() implementation function.

Rename String::AllocFromStrings() to String::DoConcat() and
rename arguments to h_this and h_arg.

This addresses a comment from
    https://android-review.googlesource.com/1473728 .

Test: testrunner.py --host --optimizing -t 021-string2
Bug: 169674485
Change-Id: I830541b64b51c0170aa5455cbad7cc86f4349f13
diff --git a/runtime/native/java_lang_String.cc b/runtime/native/java_lang_String.cc
index 7c7c553..86f9329 100644
--- a/runtime/native/java_lang_String.cc
+++ b/runtime/native/java_lang_String.cc
@@ -60,8 +60,7 @@
   int32_t length_this = string_this->GetLength();
   int32_t length_arg = string_arg->GetLength();
   if (length_arg > 0 && length_this > 0) {
-    ObjPtr<mirror::String> result =
-        mirror::String::AllocFromStrings(soa.Self(), string_this, string_arg);
+    ObjPtr<mirror::String> result = mirror::String::DoConcat(soa.Self(), string_this, string_arg);
     return soa.AddLocalReference<jstring>(result);
   }
   jobject string_original = (length_this == 0) ? java_string_arg : java_this;