Fixes the following issues:

• [iOS] Choosing the in-app "Clear Text" option (or using other text-clearing operations) had the following bugs:
  • Did not reset KMW's tracked context, causing internal desyncs that can break predictive text
  • Failed to invalidate previous predictions and generate new ones.
    • Repro: type a fair bit of text, noting the available predictions when stopping. Clear text then click a prediction, and the results (before the fix) would effectively ignore that "Clear Text" was ever selected.

The latter fix has heavy merge conflicts with other pending work unless based upon #1756, since both this PR and that one affect very similar aspects of the codebase.