Update Tor assets if they're older than the APK

Extract the Tor binary, GeoIP database and config file from the APK if they haven't been extracted since the APK was last updated.

On the Galaxy Nexus, skipping extraction of the binary if it's already up to date shaves about 1.5 seconds off the Tor plugin's startup time.

Closes #582.

Depends on !272.

See merge request !273