Add £, €, ₹ to the currency key

[BlackyHawky]

This PR improves #669 and resolves issue 666.

Keyboard languages of countries that use euro as their currency will have "€" on the associated key; the English language (GB) will have "£"; India will have "₹" and other countries will have "$" by default.

Tested on Samsung Tablet with Android 13 and on Huawei phone with Android 10.

If needed, I'll add screenshots tomorrow.

[Helium314]

I very much prefer leveraging the existing currency key logic, see 0d11066.

This doesn't duplicate code and doesn't touch subtype locales and language tags (which may have unexpected side effects).
Further, changes only need to happen in one place, e.g. when adding the Turkish Lira icon, see the todo
https://github.com/Helium314/openboard/blob/ffe7d81ebc435f9998c2c61b8f07a01099b3d284/app/src/main/res/xml/key_styles_currency.xml#L116-L117

Do you think there is anything not achieved by 0d11066?

[BlackyHawky]

Nothing to say ; it's much better than what I proposed.

I tested and found no error.

Thanks for showing me another solution.

[Helium314]

Alright, then I'll merge my variant.
Thanks for testing (and for indirectly pushing me to work on this 😉)