Amazon Pay ECE: update settings to use new payment method

[annemirasol]

Based on add/amazon-pay-payment-method

Part of #3859

Changes proposed in this Pull Request:

As part of supporting subscriptions and payment tokens for Amazon Pay, we introduced Amazon Pay's own payment method class, i.e. WC_Stripe_UPE_Payment_Method_Amazon_Pay, in #4015.

In order for the new payment method to be available to use, it needs to be present in the list of enabled payment methods. This PR updates how we enable and disable Amazon Pay as a payment method, from using a boolean flag to adding and removing it from said list.

Testing instructions

1. With the Amazon Pay feature flag off (default), verify that you do not see Amazon Pay in wp-admin or in the store.
2. Turn on the feature flag by:
   • adding _wcstripe_feature_amazon_pay with a value of yes to your wp_options table, or
   • forcing the WC_Stripe_Feature_Flags::is_amazon_pay_available() function to return true
3. Enable Amazon Pay as a payment method, in the wp-admin Stripe settings page.
4. Verify that you are able to see Amazon Pay in the product, cart (classic and block), and checkout (classic and block) pages.
5. Disable Amazon Pay as a payment method, in the wp-admin Stripe settings page.
6. Verify that you are no longer able to see Amazon Pay in the product, cart (classic and block), and checkout (classic and block) pages.

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