You should be start using "Activation event". This event is being a trigger to entry the AB test for users. Users, who triggers the activation event they will be included in measurement result and they will receive 50/50 split. If they don't trigger the activation event, they are not in AB test.

The activation event can be set in AB test detail in the section after "Targeting" section. See here on image:

Even though the values are fetched on the mobile app start, the values are activated after activation event has been triggered (this logic is upon Firebase SDK and is not documented properly).