Send empty response to IME when autofill session is destroyed

* Note that usually the session is destroyed when the focus is
  already on a new input field in a different activity, so this
  action will be no-op most of the cases
* But in case of augmented autofill service dynamically requesting
  a new autofill flow, this works well on clearing the existing
  suggestions on the current input field

Test: manual
Test: atest android.autofillservice.cts.inline
Bug: 157515522
Bug: 158038231

Change-Id: I6cedb33737d822c3c3eb4ee127984c24c4307f7a