Restore en-rGB locale for in-app strings.xml

This locale was erroneously completely removed from the constant in #17341, while in practice we wanted to only remove the `{glotpress: 'en-gb' => google_play: 'en-US' }` mapping from the locales used for Release Notes, but still needed to keep the `{ glotpress: 'en-gb' => android: 'en-rGB' }` mapping for `strings.xml` files.

fastlane/lanes/localization.rb

@@ -39,6 +39,7 @@
   { glotpress: 'el', android: 'el', promo_config: false },
   { glotpress: 'en-au', android: 'en-rAU', promo_config: false },
   { glotpress: 'en-ca', android: 'en-rCA', promo_config: false },
+  { glotpress: 'en-gb', android: 'en-rGB', promo_config: false },
   { glotpress: 'es-cl', android: 'es-rCL', promo_config: false },
   { glotpress: 'es-co', android: 'es-rCO', promo_config: false },
   { glotpress: 'es-mx', android: 'es-rMX', promo_config: false },