Autofill: allow switch IME when autofill is showing

IME used to allow startInput() *only* when window has focus.
This is no longer the case after we made changes to allow autofill
window to get window focus to receive physical keyevents.

The fix changed precondition of when InputMethodManager can startInput:
(hasWindowFocus() || isAutofillUIShowing()).

Test: manual test:
      - install two IMEs
      - install autofill sample service and autofill sample service
      - launch the Autofill sample app, click on edittext, both
        IME and autofill window are showing.
      - type "username", autofill datasets are being filtered.
      - click "globe" button in IME window to switch IME.
      - continue typing "username2" using new IME and autofill window
	continues to filtering.
      - also try the "IME switch" button in bottom bar to switch IME.
Bug: 79494235

Change-Id: I0d222b5fc13ad46834aa861647d8f2e1649093ec