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
(cherry picked from commit
d9a720bcb5df2c135f161c7c6da2d25db4400dca)
Change-Id: I0aff9c276c0f539723f8a92663f14ce72160c44d