Refactor TXPreferredLocaleProvider logic

[stelabouras]

The TXPreferredLocaleProvider class does not always report the current
locale to be used for localization correctly.

This is because the TXPreferredLocaleProvider implementation always
returns the first supported language from the Settings app, as set by
the user. That language, though, is not always the correct candidate, as
it might not be the most appropriate localization candidate, or not even
a valid candidate at all in some cases.

For example:

If user has set the following languages as their preference in the
Settings app: "Dutch", "French" and the supported languages for
localization in the app are "French", "English", "German", "Spanish"
and "Italian" then the correct language should be "French" and not
"Dutch" that is the first preferred language of the user.

Similarly, if "French" was not a supported language for localization on
the above example, then the logic must return "English" and not "Dutch".

In order to fix this issue, the TXPreferredLocaleProvider class needs
to be aware of the supported app locales (that are provided by the
developer) and also respect the order of preferred languages (that are
set by the user in the Settings app).

The method responsible for that is
Bundle.preferredLocalizations(from:, forPreferences:) and is now used
in the getCurrentLocale() method of TXPreferredLocaleProvider to
return the proper locale.

The getPreferredLocale() is now being used only as a fallback and has
been refactored for better readability and the final fallback locale
code ("en") has been made a static property (FALLBACK_LOCALE).

The TXPreferredLocaleProvider initializer now accepts the list of
supported application locales (set by the developer) as they are needed
for the aforementioned calculation.

Also, the list of the appLocales as calculated in the initializer of
the TXLocaleState has been altered so that the source locale is
always added first, in order to align with one of the internal quirks of
the Bundle.preferredLocalizations() method that fallsback to the first
language of its provided localizationsArray argument.

The testCurrentLocale unit test has been refactored to test against
the scenario where the correct locale is not the first language set by
the user and more unit tests have been added.

Ref:

• https://developer.apple.com/library/archive/technotes/tn2418/_index.html
• https://medium.com/@hectorricardomendez/ios-bad-news-bundle-preferredlocalizations-is-not-100-reliable-13fa33454d14