Clean up VLOG(oat) in OatFileAssistant.

Refactor GetOatDexFile to have an error message out parameter.

Change-Id: I6b933f1fcfc9726c051a9d9678d92587cc02501e
Test: OatFileAssistantTest with and without -verbose:oat logging turned on.
Test: m test-art-host
5 files changed