Bug 1074166 - Fix device type l10n missing issue

Regexp `return /locales[\/\\]?[a-zA-Z\/]*[\/\\]?device_type/.test(path);` makes a bad assumption that all l10n files sit under "locales" folder, but actually it's not necessary.
Let's say if we set `LOCALE_BASEDIR` to "gaia-l10n", we need to handle file path like 
`gaia-l10n/fr/apps/settings/device_type/phone/settings.device.properties`, which doesn't match original regexp on master.