ART: Check whether an oat file exists before opening

The two-stage process of dlopen and our own loader is expensive.
Considering that the OatFileAssistant needs to check two locations,
one of which is unlikely to have a file for the common case of apps
installed in data, do a file existence check before attempting to
load the file.

Remove an unused variable.

Saves about 0.25ms on a N6P.

Bug: 28801010
Change-Id: I0aff9c276c0f539723f8a92663f14ce72160c44d