Always use checkboxes for the order currency condition configuration form

I noticed today that we use a different widget for the order currency vs. the current currency conditions. For the order currency, we use our Commerce entity select widget, which converts to an autocomplete textfield when more than X currencies are enabled. For the newer condition, we just use the checkboxes widget.

On the one hand, the entity select widget is nice for avoiding massive, unwieldy forms. However, in this context, there are relatively few sites in the world that are going to have every currency enabled, while the default threshold for reverting to an autocomplete is relatively low for some merchants. Additionally, the full checkboxes list is not always going to be visible - only when the conditions are being edited.

My proposal is to standardize the order currency condition to use checkboxes, but we don't have consensus, we should at least revise the current currency condition to use the same entity select widget.