ECE: enforce separate location settings

[annemirasol]

Fixes STRIPE-286
Fixes #3935

Changes proposed in this Pull Request:

• Fixes the issue where the Amazon Pay button shows up inside the Customize page for Apple Pay/Google Pay
• Fixes the issue where location settings for Amazon Pay and Apple Pay/Google Pay can affect each other.

Testing instructions

1. Enable the Amazon Pay feature flag: wp option update _wcstripe_feature_amazon_pay yes
2. Enable Amazon Pay express checkout.
3. Go to Customize, and enable it only for Checkout.
4. Enable Apple Pay/Google Pay.
5. Go to Customize, and disable it for Checkout.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
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