Clean up the payment method plugin forms

Found a number of problems while reviewing #2905028: Add a per-gateway setting to skip collecting billing information:

1) PaymentMethodAddForm and PaymentMethodEditForm use Profile:: even though the entity type manager is injected.
2) PaymentMethodAddForm has route-or-default-store logic which duplicates the resolved store logic. We should inject the currentStore service and get the store from there instead.
3) PaymentMethodEditForm doesn't validate the newly entered CC expiration date.

Plus, we could use a PaymentMethodFormBase, which injects the common dependencies and handles the billing information.