fix(web): simplify hide transition

Fixes #5907.

The state machine for the Hide transition for the OSK was very complex,
and had some peculiarities which caused it to sometimes fail to
correctly hide. Once I simplified the state machine, it seems that those
edge cases have gone away, and the keyboard always hides correctly.

There is an unrelated issue with focus timeout which can be confusing
when testing this; see #5909.