Fix fingerprint set error on legacy checkout

[wjrosa]

Fixes #3718

Changes proposed in this Pull Request:

This PR fixes a fatal error being thrown when saving payment method tokens with the legacy checkout enabled. The error happens when setting the fingerprint property (used to identify duplicate tokens). The expected object structure was different from the actual format received (fingerprint exists inside the payment method details key, source).

Testing instructions

You can follow the test instructions from the related issue:

1. Configure a test WP install with the WC, WC Stripe, and WC Subscriptions plugins
2. Enable Stripe legacy checkout
3. Add a credit card via the "Payment Methods" page
4. Purchase a subscription, entering a different credit card during checkout
5. Using a database admin tool, delete the first credit card you added from wp_woocommerce_payment_tokens to simulate the case where a card is defined in Stripe but not synced to the WC database
6. Attempt to add another card via the "Payment Methods" page
7. Confirm no fatal errors are thrown

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[diegocurbelo]

The code changes look good, and testing by deleting the first credit card from the db works as expected.

@wjrosa I'm approving this, but let's revert the spacing fixes/changes before merging.

I'll open a PR to fix the linting checks (and the 19 files that need changes)