Multifield clozes

Use `replaceAll()`, not `replaceFirst()` when building cloze cards where Anki desktop [uses](https://github.com/dae/anki/blob/3ee193731007ca8df28cbe9534218a70d97599fa/anki/collection.py#L514) `replace()`.
This should fix issue [#1446](https://code.google.com/p/ankidroid/issues/detail?id=1446).