Use canonical paths when searching for dex files

Apps which use the DexPathClassLoader directly may
pass symlinks when trying to load dex files. This
will not work as we use string comparision to find
the dex in an oat file. The CL fixes this issue by
using using dex conical paths for comparisons.

Bug: 15313272

(cherry picked from commit 4e1d579d)

Change-Id: I441f1ef18388c4a17c747a7e55b57f917724db85