Transifex exports empty android strings

When you export a the localized strings.xml files from Transifex, untranslated strings are included as empty string resources.

As an example, in the default english strings you might have:

Start Tor

and then in another language, say Spanish, if that string is not translated in the transifex UI, you would have:

By exporting an empty string resource, Android will use that empty string in the user interface, instead of gracefully degrading to the default english value.

This currently means we can only include translations from Transifex that are 100% complete, which is not what we have done in the past via pootle.