Upgrade AGP to 8.9.1

[mboetger]

Upgrades AGP to 8.9.1. A few dependabot PRs are stuck because they are failing with similar error messages:

* What went wrong:
Execution failed for task ':app:checkDebugAarMetadata'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
   > An issue was found when checking AAR metadata:

       1.  Dependency 'androidx.browser:browser:1.9.0' requires Android Gradle plugin 8.9.1 or higher.

           This build currently uses Android Gradle plugin 8.6.0.

Blocked PRs:

• [dependabot]: Bump androidx.core:core from 1.13.1 to 1.17.0 in /packages/url_launcher/url_launcher_android/android #9855
• [dependabot]: Bump androidx.activity:activity from 1.10.1 to 1.11.0 in /packages/image_picker/image_picker_android/android #10045

Pre-Review 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].
• 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 I have commented below to indicate which [version change exemption] this PR falls under[^1].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style], or I have commented below to indicate which [CHANGELOG exemption] this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request updates the Android Gradle Plugin version from 8.6.0 to 8.9.1 in .ci/legacy_project/all_packages/android/settings.gradle. My review identifies a potential issue where a corresponding Gradle version update might be necessary to ensure compatibility with the new AGP version, which could otherwise lead to build failures.

[stuartmorgan-g]

LGTM, but please update the project change description to match.