Change the way we decide whether we want to show on-screen keyboard by
not only paying attention to modifiers, but also keeping track whether
the key sequence started in the right state.

We are still misfiring if user presses a non-modifier key and then our
modifier hot-key, but such sequence is unlikely. Given the fact that we
do not want to store too much state I believe this deficiency is
acceptable.

Bug: 25087681
Bug: 24142161

Change-Id: I1a6b5e8e903c27a87134a6c9a7cd474a0607d5c8
(cherry picked from commit 7c513455918a52bd28c1c8181cb2880db0973b4b)