Fixes a gap that appeared under the keyboard after switching the action
bar on or off.

The strip kept for the system's own bottom buttons is measured from the
window's insets, and switching that setting rebuilds the keyboard while
the settings screen - not the keyboard - is the thing on screen. The
measurement taken then belonged to the settings screen, and it stuck.

The keyboard now measures again whenever its own window becomes visible,
so the answer is always about the window it is actually about.
