Line items display system default currency, not selected currency

1.0 Problem/Motivation

If you have a site with a default currency (e.g. USD) and you create a form that accepts a different currency, (e.g. GBP), the currency is formatted correctly on any elements that involve currency (e.g. `civicrm_1_contribution_1_contribution_total_amount`). However, `displayLineItems()`, on the payment page, will show the currency as your default.

2.0 Detailed steps to reproduce (embed screenshots)

* Create a form that has a different default currency than your own.
* Select an amount to pay.
* Go to the payment screen.

3.0 Proposed resolution

PR incoming.