For automatic topic subscriptions, I plan to introduce a third
subscription state to indicate them. This patch includes minor tweaks
I wanted to add while working on that:
* Introduce constants instead of numbers
* Remove a TODO that doesn't seem like a good idea any more
* Remove a `"length": 1` on sub_state that did not do anything
(but it might have been meant to indicate that it was supposed
to be a boolean, which would be wrong)
Bug: T284836
Change-Id: I6e6096968ad38510102287bccd349090b6ca4280
