Issue 1743: Use US locale when formatting DB queries.

[Issue 1743](https://code.google.com/p/ankidroid/issues/detail?id=1743)

Syncing does not work when certain system languages like Arabic are in use. In the syncer code, there are three places where we use String.format without specifying a locale. [The documentation](http://developer.android.com/reference/java/util/Locale.html#default_locale) is pretty clear about not using the default locale for machine-readable text. We end up building a bad query like this:

<tt>android.database.sqlite.SQLiteException: no such column: ٢٧٩٢ (code 1): , while compiling: SELECT id, cid, ٢٧٩٢, ease, ivl, lastIvl, factor, time, type FROM revlog WHERE usn = -1</tt>

This change makes use of the US locale, as recommended by the API documentation.