Store the weave_settings by model manifest id

Create different weave_settings.json files for each model manifest id.
Since model manifest id is immutable on the server for a device id, this
prevents the model manifest id for a sample device from changing

Change-Id: I33ad00271efeca2f81ac4e1dcd27198cb1c576ca
Reviewed-on: https://weave-review.googlesource.com/1844
Reviewed-by: Alex Vakulenko <avakulenko@google.com>