Restructure public side of geometry service.

This CL goes from this:
  //mojo/services/public/cpp/geometry
  //mojo/services/public/interfaces/geometry

to this:
  //mojo/services/geometry/public/cpp
  //mojo/services/geometry/public/interfaces

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

TBR=beng

Review URL: https://codereview.chromium.org/790213002