Use class def index from java.lang.Class.

Bug: 10244719
Depends on:
https://googleplex-android-review.git.corp.google.com/362363
This removes the computation of the dex file index, when necessary this is
computed by searching the dex file. Its only necessary in
dalvik.system.DexFile.defineClassNative and DexFile::FindInClassPath, the
latter not showing up significantly in profiling with this change.

Change-Id: I20c73a3b17d86286428ab0fd21bc13f51f36c85c
diff --git a/runtime/mirror/art_method-inl.h b/runtime/mirror/art_method-inl.h
index 224b2ba..ccf3e59 100644
--- a/runtime/mirror/art_method-inl.h
+++ b/runtime/mirror/art_method-inl.h
@@ -178,7 +178,7 @@
 }
 
 inline bool ArtMethod::IsRuntimeMethod() const {
-  return GetDexMethodIndex() == DexFile::kDexNoIndex16;
+  return GetDexMethodIndex() == DexFile::kDexNoIndex;
 }
 
 inline bool ArtMethod::IsCalleeSaveMethod() const {