Apps2SD fixes.

Better recovery after a data wipe.

* Prune stale dex files from /sd-ext/dalvik-cache
* Reset permissions on forward-locked packages when no data directory
  exists (the "fix_permissions" use case)

This does not address the case where a user has protected packages on
sd, and the uids of the package files on disk do not match existing
data directories. This situation is more difficult to get into, but
still needs consideration.