As CL[1] removed InputMethodService#onToggleSoftInput since framework
side changed internal protocal to simplify the toggle logic without
this method callback.
Also remove ClientCallbackImpl#onToggleSoftInput for
MultiClientInputMethod sample app part to sync this change.
[1]: I390dc029e7bcc30c200926a9bfbbbd0268a1f714
Bug: 182071625
Test: build MultiClientInputMethod
Change-Id: I25568d9d0a963562fdc1d4a021f7016689b9ca54
9b36e56849