Pods created due to initialScale doesn't follow scaleDownDelay behavior

## In what area(s)?
/area autoscale

## What version of Knative?

0.19.0

## Expected Behavior

Pods created due to `initialScale` should follow `scaleDownDelay` behavior.

## Actual Behavior

Pods created due to `initialScale` doesn't follow `scaleDownDelay` behavior.

## Steps to Reproduce the Problem

Create a revision with the following configuration:
```
autoscaling.knative.dev/initialScale: "3"
autoscaling.knative.dev/scaleDownDelay: "10m"
```

Check that three pods are created, but two of them are terminated right after the revision is ready, which will be likely too soon for a `10m` `scaleDownDelay`.

No source file or test is named in the issue. Reproduce a revision with initialScale set to 3 and scaleDownDelay set to 10m, then trace the autoscaling path that terminates the extra pods after readiness. Done means pods created by initialScale respect the configured scaleDownDelay; add coverage for this timing behavior.

Written by the indexing model from the issue text.