[APM] APM agent config created prior to Fleet migration is not injected into integration policy

[axw]

Kibana: 7.14-SNAPSHOT

If you create APM agent config in the UI, and subsequently run the Fleet ("schema") migration, then the agent config is not injected into the integration policy initially.

Steps to reproduce:

1. Create a 7.14-SNAPSHOT deployment in cloud QA
2. Navigate to the APM app in Kibana, create an agent config for "All"

3. Still in the APM app, run Fleet/schema migration ("Switch to data streams")
4. Navigate to the Fleet app, view the integration policy; it should have the agent config included, but will be missing

See also elastic/apm-server#5177 (comment)

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)