ART: Allow oat files with duplicates classes in corner case

When the oat file is actually an odex file, that is, a preopted
/system app, then it is impossible to fall back to the original
APK, as that has been stripped.

When it looks like it will be impossible to successfully open the
original dex location, grudgingly allow to open the found oat file,
even if it has duplicate classes, but warn accordingly.

Bug: 20697582

(cherry picked from commit 0cba004b)

Change-Id: I1dd459563d977a2e77806eacd03e49334d5b1f14