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 8b2c0b9a)
Change-Id: I20c73a3b17d86286428ab0fd21bc13f51f36c85c