Invalidate autofill session on IME disconnect

Remember the latest InlineSuggestionsRequestCallback coming from the
autofill service, so that its autofill session can be invalidated
when the IME process is unbound or stopped.

Test: atest InlineWebViewActivityTest#testAutofillOneDataset
Bug: 226240255
Change-Id: I49cf295b95cec33b7eb404eab62b900014d09e26