Restructure public side of view_manager service.

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

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

This CL also makes the Mojo-side changes required to roll this change into
Chromium (for both view_manager and window_manager, which was converted in an
earlier CL but for which these updates were not made):
- Updates rev_sdk.py to pull over the new directory
- Updates //mojo/services/public/mojo_services_public.gyp

TBR=beng

Review URL: https://codereview.chromium.org/790623003