Update Gradle Command Test to Only Accept Gradle Declarative Apply

[jesswrd]

Currently the tests accept both the old gradle imperative apply and the gradle declarative apply. Since all plugins have been migrated to the gradle declarative apply, we want all exmple app plugins to reflect that and only accept gradle declarative apply.

Fixes flutter/flutter#152656
Fixes flutter/flutter#157660

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[jesswrd]

@reidbaker Requested review with a failing test, but it looks unrelated to my changes. It's from tip of tree.