Use real locations when opening dex files from the context

The oat file assistant uses real locations when opening dex files which
will later be checked against the class loader context embedded in the oat
file. In order to guarantee a match we need to use real paths in the
context as well.

Bug: 64460009
Test: m test-art-host-gtest
Change-Id: I1b564baa0c933172891a10a448ce1ef129be5ea2