Pass OatDexFile by reference in FindDexCache

This is more robust as we were passing pointers before and we could
potentially get an unintended dex cache.

As a drive-by, improve logging until bug 206992606 has been fixed.

Bug: 206992606
Test: ART tests
Change-Id: I98f309a27fd1c502199513f657c0cef65a1f0eb3
3 files changed