Bug 1832726 - part 1: Make Android widget always dispatch keyboard events during composition r=m_kato,geckoview-reviewers

I think that we don't need the prefs to disable the behavior because the
behavior conforms to UI Events definition and we've shipped the behavior in
2018.  Therefore, nobody shouldn't require to disable the behavior.

Differential Revision: https://phabricator.services.mozilla.com/D177999