Cache filtered enabled payment methods in BTPaymentButton
Cache filtered enabled payment methods in BTPaymentButton

- Fixes acceptance test for drop in
- BTPaymentButton shows UI for payment types that are enabled and
  available when it is first initialized; when it processed taps, it
  would resolve the payment type by checking for which payment types
  are enabled and available again, which caused a mismatch when the set
  of enabled and available payment types had changed.

braintree/braintree_ios,apascual/braintree_ios,bytemarkinc/braintree_ios,apascual/braintree_ios,Feverup/braintree_ios,EverettQuebral/braintree_ios,billCTG/braintree_ios,apascual/braintree_ios,apascual/braintree_ios,bytemarkinc/braintree_ios,billCTG/braintree_ios,Feverup/braintree_ios,bytemarkinc/braintree_ios,bytemarkinc/braintree_ios,bytemarkinc/braintree_ios,EverettQuebral/braintree_ios,billCTG/braintree_ios,EverettQuebral/braintree_ios,Feverup/braintree_ios,braintree/braintree_ios,Feverup/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,EverettQuebral/braintree_ios,EverettQuebral/braintree_ios