Android Gradle Plugin 9.x is available — major upgrade (manual)

[github-actions]

A new major Android Gradle Plugin release is available.

Currently pinned	8.10.1
Newest stable	9.4.0
Newest stable on the pinned major	8.13.2
Pinned in	android/settings.gradle
Source	https://dl.google.com/dl/android/maven2/com/android/tools/build/gradle/maven-metadata.xml

No PR was opened, on purpose

android/settings.gradle has not been changed and no commit was made. Patch and
minor updates within the pinned major get an automatic draft PR; a major
does not, because a major upgrade is not a one-line pin change (#362).

The two are tracked separately: if the newest release on the pinned major
is ahead of what is pinned, that update still gets its own ordinary draft
PR this week. A new major never blocks it.

Read this one carefully

Linthra's F-Droid release still builds its per-ABI versionCodes by
iterating applicationVariants in android/app/build.gradle. That is
legacy Android DSL behaviour, and the next AGP major removes the API it
depends on. Bumping this pin without first porting that logic does not
produce a build failure so much as a wrong versionCode scheme —
which is the one thing an F-Droid release cannot get wrong, because
published version codes can never be reused or walked back.

Treat the variant/versionCode port as the actual work item here. The
version bump is the easy part.

Why this stays manual

• Flutter compatibility. Flutter ships its own Gradle plugin and
  supports a bounded range of build tooling. The SDK pinned in
  .flutter-version decides what this project may actually use, and
  "newer" is not the same as "supported".
• The Android build itself. A major here changes defaults, removes
  deprecated DSL and can alter what ends up in the APK. That is a review,
  not a version bump.
• F-Droid and reproducibility. The F-Droid builder resolves this
  toolchain offline from the files in this repository. A major upgrade can
  change what the builder must provide and whether the build still
  reproduces, and CI cannot prove either. See
  docs/fdroid-build-recipe.md and the reproducibility notes.
• The rest of the toolchain. Gradle, AGP, Kotlin and the JDK move as a
  compatible set; one of them crossing a major usually drags the others.

Suggested route

Do the upgrade in a normal human PR: check Flutter's supported range
first, bump the pin, port whatever the major removed, re-check the
F-Droid recipe and reproducibility notes, and let normal CI judge it.

Filed by android-toolchain-updates.yml
for issue #362. This issue is updated in place on later runs rather than
refiled, so the weekly check will not spam duplicates.