Fix: express checkout displays wrong amount for JPY

[Mayisha]

Fixes STRIPE-494

When the store currency is set to JPY, the express checkout modals/pages display 100x the amount. This only happens on the shortcode cart and shortcode checkout pages.

Changes proposed in this Pull Request:

We are using the currency_decimals field to calculate price in the frontend but its value is never set on the backend. So by default 2 is used for every currency during the calculation.
This PR sets the currency_decimals in params to be used in the frontend.

Testing instructions

• Set store currency to JPY and set the Number of decimals to 0.

• As a shopper, add a product to your cart and go to the shortcode checkout page.
• In develop, click the Link button and notice that the amount displayed is 100x more than the actual total price.

• In this branch, click the Link button and confirm that the correct amount.

• Check Google Pay and Apple Pay modals and confirm that they are showing the correct amount as well.
• Purchase the product with express checkout and confirm that correct amount has been charged in Stripe.
• Change the store currency to USD/Euro and Number of decimals to 2. Confirm that the amount displayed in express checkout is correct for this currency.

[annemirasol]

Code LGTM!

• Was able to reproduce the issue*
• Verified this PR fixes it

Before	After

* I can only reproduce the issue when my Link is logged out. After logging in, it fixes the price. When logged in, the price is always correct.

In develop:

Logged out	After logging in