Prevent keypresses from triggering onNormalizedDisplayChange

This avoids toolbar buttons from becoming disabled in IE11 when
the editor frame is unfocused by keyboard navigation to the
toolbar. The onNormalizedDisplayChange will occur once the entire
editor is blurred.

The ultimate solution is probably to stop using
queryCommandEnabled (like all other RTEs) but that would require
a significant amount of additional effort for diminishing gain.

Fixes #17595.