Fork SoftInputWindow into VoiceInteractionWindow

Since its beginning [1], VoiceInteractionService has been using
android.inputmethodservice.SoftInputWindow to host its UI, which is
understandable given the successful history of InputMethodService.

Fast forward 7 years, InputMethodService still keeps evolving.
Features like stylus support and in-proc navigation bar buttons are on
its radar.  It's getting more and more likely that SoftInputWindow
needs to be updated to support new IME features.

In order to avoid accidental behavior changes in
VoiceInteractionService, and also to keep SoftInputWindow simple and
maintainable, this CL creates a copy of SoftInputWindow as
VoiceInteractionWindow so that each service can maintain its own
Window class.  Hopefully this is beneficial for both the IME team and
VoiceInteraction team.

This is a mechanical refactoring CL.  Hence there should be no
observable behavior change.

 [1]: Ie2b85b3020ef1206d3f44b335b128d064e8f9935
      e6e6b086d6b641de79246413f6d885e1f7d06386

Bug: 192412909
Test: presubmit
Change-Id: I0054dd1bdcc93af499832de00c76b4578f48e627