Braintree - Drop-In UI should default to "on"

[angelleye]

In order for PayPal/Braintree to approve us we have to tie PayPal in using the Braintree SDK instead of Express Checkout APIs.

The bare minimum thing we can do in order for them to certify our plugin is to make the Drop-In UI enabled by default. So we need to go ahead and do that here.

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In #546 we'll be adding PayPal payments using the Braintree SDK so then people can leave the drop-in UI disabled, but still use PayPal Shortcut payments from the product/cart pages without needing to setup 2 separate gateways.