Hide the overflow when the IME comes up sometimes

If there isn't enough space to show all the bubbles
when the IME is up on large screens, then hide the
overflow.

Really the overflow isn't hidden, it just doesn't
get shifted up above the IME like the actual bubbles.

Made a little helper class to hold stack state info
and pass that around.

Test: manual - on large screen that can't fit max #
               of bubbles when IME is up, select the
               last bubble & check that IME shifts
               all the bubbles but the overflow
Bug: 193911220
Change-Id: I74b0d66cedf70cca40a36b07933ebf6466da63b7