Use class def index from java.lang.Class.

Bug: 10244719
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.

(cherry-picked from 8b2c0b9abc3f520495f4387ea040132ba85cae69)
Change-Id: I20c73a3b17d86286428ab0fd21bc13f51f36c85c
diff --git a/runtime/oat_file.h b/runtime/oat_file.h
index 325ebb2..bbd2615 100644
--- a/runtime/oat_file.h
+++ b/runtime/oat_file.h
@@ -183,7 +183,7 @@
     }
 
     // Returns the OatClass for the class specified by the given DexFile class_def_index.
-    const OatClass* GetOatClass(uint32_t class_def_index) const;
+    const OatClass* GetOatClass(uint16_t class_def_index) const;
 
     ~OatDexFile();