Simple payments block: default currency to user's preferred choise

When creating a Simple Payments block in Gutenberg, the currency selection now always defaults to USD.

Currency should default to user's locale just like it does in Calypso Classic editor:

The currency could also default to _site's_ locale.

It could also default to previously chosen currency if available so that user wouldn't need to choose it for each block separately.

Locate the Simple Payments block's currency-selection entry point and compare its defaulting behavior with the Calypso Classic editor. Resolve whether the default should use the user's locale, the site's locale, or a previously selected currency, then verify that a newly created block no longer always selects USD.

Written by the indexing model from the issue text.