...otherwise we can't recompute the composition when we
change scripts.

This also fixes when we register that we need to take note
that the current subtype was used. Luckily this is a good
occasion for some cleanup that I've wanted to do for some
time: use InputTransaction for onTextInput (with the goal
to ultimately remove it entirely)

Bug: 15840116
Change-Id: Ie4f4f9157b66b79237eeb3db75535803124d3e19