feat(ios): Improve system keyboard load performance

**Is your feature request related to a problem? Please describe.**
The app has received a number of crash reports (see #1169, #1170, #1172) that are caused by poor system-keyboard startup performance. If a keyboard app-extension takes too long to load, iOS explicitly kills it.

**Describe the solution you'd like**
As documented on #1169, profiling `SWKeyboard`'s startup time should yield some insight on what components could be sped up, delayed, or performed asynchronously in order to promote responsiveness, both to the user and to the system.

**Describe alternatives you've considered**
Apple doesn't really give the app alternatives.

**Product context**
iOS

**Additional context**
#1475 was a good first step toward addressing these issues.


Based on the final profiling obtained during that work, finding a way to streamline `Manager`'s load time, perhaps by reducing the amount of data necessary to retrieve from 'app group' space at startup, would probably be a decent place to start. Less important but also possible would be to delay creation of the WKWebView, though that may have UI/user impact.