Use emoji flags directly instead of computing them

[minhdanh]

I'm experiencing a noticable delay when opening the currency picker widget. I guess because it has to compute more than 200 flags when generating the listview. Can we just add the emoji data directly to the list? I think that will boost the performance of the package. What do you think?

[Daniel-Ioannou]

@minhdanh Yes we can, feel free to create a PR.