1875336 - [meta] Migrate isEarlyStartup features to use setPref instead

Status: NEW

(Firefox :: Nimbus Desktop Client, task, P5)

Description

[Beth Rennie [:beth] (she/her) - OOO until Sept 14]

We can migrate each isEarlyStartup feature to use setPref on the computed nimbus.sync{data,defaults}store.<feature>.<variable> pref easily. However we also have to migrate the enrollment database to add pref entries for any live experiments and rollouts. This should be done in ExperimentManager#onStartup, before the enrollment prefs are restored.

This migration will likely have to stick around for a while and we have two options:
a) keep the isEarlyStartup flag in the feauture description even after it has no use, or
b) move the list of isEarlyStartup features into the migration code so it is clear to consumers it has no use.