[Android] Add Flutter 3.50 breaking-change pages for build tool minimums and the Flutter Gradle Plugin Variant API migration - #14053

Description of what this PR is changing or adding, and why:

Adds two breaking-change pages that Android developers need before
upgrading to Flutter 3.50 (stable release planned for 2026-11-12), and lists
them under "Released in Flutter 3.50" in the breaking-changes index.

1. Raised minimum Android build tool versions
   (/release/breaking-changes/android-build-tool-minimum-versions)
   • Flutter 3.50 raises the minimum AGP to 9.1.1 (from 8.11.1),
     Gradle to 9.3.1 (from 8.14.0), and KGP to 2.4.10 (from 2.2.20,
     only when KGP is applied). Template defaults are AGP 9.3.1 /
     Gradle 9.5.0 / KGP 2.4.20. AGP 10+ is rejected with a new error.
   • Calls out that the AGP minimum (9.1.1) is above the version Flutter 3.47
     warned about (9.0.1), so AGP 9.0.1–9.1.0 users get no advance warning.
   • Explains how built-in Kotlin (android.builtInKotlin=false) and the
     new AGP DSL (android.newDsl=false) properties are handled when a
     project moves to AGP 9 as part of this upgrade, and links to the existing
     built-in Kotlin guides for app developers and plugin authors.
   • Documents that KGP validation is skipped for built-in Kotlin projects,
     that version checks now run first, and the bypass flag.
2. Flutter Gradle Plugin uses the Android Gradle Plugin Variant API
   (/release/breaking-changes/flutter-gradle-plugin-variant-api)
   • Publishes the user-facing content of
     docs/platforms/android/website-page-draft.md,
     adjusted to the state actually shipping in 3.50: the FGP itself no longer
     uses the legacy variant API (applicationVariants, libraryVariants,
     versionCodeOverride, AGP internals), but android.newDsl=false is
     still set by templates and the migrator, so users' own legacy build
     script code keeps working in 3.50. The page tells them to migrate now.
   • User-visible behavior changes: copyFlutterApks<Variant> task,
     per-ABI versionCode computed from the DSL (with the "multiply by
     10000" worked example), custom build types copied with initWith
     (native debuggability caveat), Flutter assets as a generated source
     directory, lazy task registration / CopyFlutterAssetsTask type,
     add-to-app changes (flutter.hostAppProjectName no-op, build mode
     follows the consumed module variant), and flutter build aar
singleVariant behavior.
   • Migration guide with Kotlin DSL examples for APK renaming,
     per-ABI versionCode, custom build types, task lookup, add-to-app
     hosts, and plugin authors, plus the temporary android.newDsl=false
     escape hatch.

Facts were verified against the flutter-3.50-candidate.0 branch
(DependencyVersionChecker.kt, gradle_utils.dart, FlutterPlugin.kt,
templates, and migrations), the merged PRs listed in each page's
References section, and an independent fact-check pass whose corrections
are in the second commit.

Follow-ups once this lands (tracked in flutter/flutter#193713): point the
FlutterPlugin.kt KDoc and the contributor doc at the published URL, and
reduce website-page-draft.md to a pointer.

Issues fixed by this PR (if any):

• Fixes Publish the Flutter Gradle Plugin AGP public API breaking-change page to docs.flutter.dev and update links to it flutter#193713

PRs or commits this PR depends on (if any):

None. Intended to land with the Flutter 3.50 stable release.

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