Fix missing IME switcher icon (w/ a hardware keyboard)

This is a follow up CL to our previous CL [1], which aimed to adjust
the timing to change navbar icons that depend on the IME window
visibility.

One thing we overlooked is that only InputMethodService#IME_VISIBLE
needed to be cleared to keep the back button icon to behave as if the
IME window was invisible.  For the IME switcher icon to work as
intended, the current protocol requires InputMethodService.IME_ACTIVE
to remain to be set even when an IME window is invisible as long as
there is an active session between the app and the IME.

 [1]: I4dc9d6513d0559156f7da39244f3fc5ebc952ed4
      c22eec9d

Fix: 160283619
Test: Manually done as follows:
  1. Build aosp_coral-userdebug and flash it
  2. make -j SoftKeyboard
  3. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  4. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  5. adb shell ime set com.example.android.softkeyboard/.SoftKeyboard
  6. Open any app that has an EditText
  7. Focus in to the EditText. Make sure the IME shows up and the IME
     switcher icon is visible on the navbar.
  8. Attach a hardware keyboard. Make sure that the IME becomes hidden
     while the IME switcher icon remains to be visible on the navbar.
  9. Detach the hardware keyboard.  Make sure that the IME switcher
     icon remains to be visible on the navbar.
Change-Id: I4a9ddf514a32a221c2372be0ca166fbdac555932