Wait on consumer_info promise in login (bug 1038985, bug 1039868)

https://bugzilla.mozilla.org/show_bug.cgi?id=1038985
https://bugzilla.mozilla.org/show_bug.cgi?id=1039868

We need to wait for consumer-info because it's the source of the waffle switches - if we don't, then sometimes the `fallbackFxA()` check is done too early and returns false when it's enabled.

TL;DR: we should really fix [bug 1038936](https://bugzilla.mozilla.org/show_bug.cgi?id=1038936) and remove the switches from consumer_info, it creates a ton of subtle problems.