Revert "Use more devices when doing firebase tests."

This reverts commit 227ef49500d09284cb4bd2e410af024b0566c1d7.

Reason for revert: Failures like https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8858650554933690528/+/steps/test_execution/0/steps/gcloud_firebase/0/stdout

Original change's description:
> Use more devices when doing firebase tests.
>
> This uses virtual devices to avoid availability/latency issues. The
> intention of this patch is to give us more API level coverage on Android.
>
> Bug:https://github.com/flutter/flutter/issues/49095
> Change-Id: I0f9a75b13767ec1bc50470086df9b0f40cbbbe5e
> Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/9620
> Commit-Queue: Dan Field <dnfield@google.com>
> Reviewed-by: Godofredo Contreras <godofredoc@google.com>

TBR=godofredoc@google.com,flutter-scoped@luci-project-accounts.iam.gserviceaccount.com,dnfield@google.com,egarciad@google.com

Change-Id: Id42945d69dd5a3c9f00eb34e2e3633bbd2ef5979
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: https://github.com/flutter/flutter/issues/49095
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/9660
Reviewed-by: Dan Field <dnfield@google.com>
Reviewed-by: Gary Qian <garyq@google.com>
Commit-Queue: Dan Field <dnfield@google.com>