fix(ios): call keyman.resetContext, not keyman.core.resetContext

The iOS code was making a direct call to `keyman.core.resetContext()`
rather than to `keyman.resetContext()`. This meant that the
`OutputTarget` was not set and the `newContext` event would never be
called (as well as triggering a background warning in the language
processor "OutputTarget missing during an invalidateContext call").

Refactored to include a wrapper for `resetContext()` in ios-host.js, as
I think it's better to proxy all calls to KMW consistently through the
host javascript.

(There are a couple of other direct calls which could be cleaned up;
search for "evaluateJavaScript" to find all calls; I think we could
consider build a wrapper for all these calls?)