Clarify state names for IME Switcher button

We had a few instances of properties referencing the IME Switcher or its
visibility, when in fact this was related to the button, not the menu.

This clarifies the names for these states, as well as adding some
documentation around where these values are used/passed.

Flag: EXEMPT bugfix
Bug: 366129400
Test: n/a
Change-Id: Ia75cdbfcd0cf4642bc747a132c8500908c869885