Restructure public side of input_events service.

This CL goes from this:
  //mojo/services/public/interfaces/input_events

to this:
  //mojo/services/input_events/public/interfaces

This CL also makes the Mojo-side changes necessary to roll this change into
Chromium.

TBR=beng

Review URL: https://codereview.chromium.org/788353002