IME switcher should not always require IME_VISIBLE.

When one ore more physical keyboards are attached, we should not require
InputMethodService.IME_VISIBLE to show the IME switcher because some
IMEs such as LatinIME shows no software keyboard w/ physical keyboard.

Bug: 26245853
Change-Id: I8aac9f5989ebaf544ace06a3ba0993b3a834869d