Add constants for schema migration feature flags

Add constants for schema migration feature flags

A general behavior is to progress through four stages:
* Access only the old schema (until the schema change is deployed to the
  databases)
* Read and write both schemas (for easy rollback while testing)
* Write only the new, read both (while a maintenance script runs to
  migrate existing old schema to the new)
* Access only the new schema (then we can drop the old)

This adds constants for use with feature flags controlling these stages.

Bug: T166732
Bug: T167246
Change-Id: I0fb637ca8c10ec91d6eda706d9d8a5ce9be41a06