extract_utils: avoid doing stuff on files that don't exist

* If an apk/jar doesn't exist, the script would still try to deodex it.
* If an xml doesn't exist, the script would still try to "fix" it.
* Take it easier, man, it's not your fault.

Change-Id: I3061fb48b403da5121e3c17dd9ecdb6cd148bf97
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>