Deprecate InputMethodService#onViewClicked()

This CL marks InputMethodService#onViewClicked() deprecated.  There is
no actual implementation change at this moment, but developers should
avoid relying on this callback in general not only on futuer versions
of Android but also on existing devices.

Since this API was implemented in Android 4.0 ICS [1] it never worked
well for WebView and any other UI libraries that expose themselves as
a giant "Canvas", which can host their own UI hierarchy and sub focus
state.

If IME developers want consistent and reliable user interaction across
devices and applicactions, implementing IMEs' own UI on top of
CursorAnchorInfo API would be a good choice.

If IME developers just want to be notified an existing input
connection is interrupted, they can just monitor
InputMethodService#onFinishInput().

 [1]: Ia71ad325e50a20e9e1001def662ef992bb50a9d0
      863fcd62

Fix: 123245358
Test: make -j checkbuild
Change-Id: I37a6c6d82d4a424f0df84633b09bca4881efbf02