Fix dex file pruning logic.

We should now prune all normal files from /data/dalvik-cache
in addition to looking for dex files in all subdirectories of
/data/dalvik-cache.

(cherry picked from commit 51a6f9253399588eedf77d75c578d9aa23d11529)

Change-Id: I536dfdc48e94155e7be64eb4efd9f7f2a1d2d00a