Stop sampling when Reference Previews is enabled

There are so few users with this configuration that we need to stop
sampling in order to get data. Sampling is still in effect for
"baseline" users with the feature disabled.

We use separate schemas to simplify analytics on the two populations.

Bug: T214493
Change-Id: I16e4ed236e50e1e246ff28ff0dba3e52e4b56caf