Bug 1673669 - Fix Address Bar IME composition interaction with Search Mode and the Event Bufferer. r=adw

Fix a race condition with Korean IME, where it may submit delayed text after
Enter has been handled. The patch delays events if composition is still ongoing
and avoids using the selected element or the last resultForValue if it's not
yet complete by the time we submit the text.
Unfortunately this cannot be tested automatically in mochitests because it
depends on a native behavior of the IME that synthesize methods can't reproduce.

Additionally this fixes Bug 1679697 by ensuring Search Mode is not dismissed
by IME and the urlbar value is proper when confirming Tab-to-search.
This part comes with tests.

Differential Revision: https://phabricator.services.mozilla.com/D98768