fix: update Gradle 8.12 + AGP 8.9.1 for JDK 23+ compat

[GhagSagar23]

Summary

Updates all 22 Android sample directories to Gradle 8.12 and AGP 8.9.1, fixing builds on JDK 23+.

Changes per sample category:

• All 22 samples: gradle-wrapper.properties → Gradle 8.12-all
• 6 hybrid Flutter samples (animations, form_app, navigation_and_routing, platform_channels, platform_design, asset_transformation): AGP 7.3.0 → 8.9.1 in settings.gradle, added Kotlin 2.1.0 plugin, removed redundant buildscript block from build.gradle
• 5 old-style Flutter samples (google_maps, material_3_demo, simple_shader, testing_app, pedometer/example): full migration from buildscript/apply plugin to pluginManagement + plugins {} DSL, added namespace, removed package= from AndroidManifest.xml
• 2 native add_to_app samples (books/android_books, fullscreen/android_fullscreen): AGP → 8.9.1, Kotlin → 1.8.22, apply plugin → plugins {}, added namespace where missing, compileSdk/targetSdk → 36
• 1 Flutter plugin (pedometer/android): AGP 7.3.0 → 8.9.1, added namespace

Why Gradle 8.12 + AGP 8.9.1:

• Gradle 7.x caps at JDK 18-19; JDK 23 produces Unsupported class file major version 67
• Gradle 8.12 supports JDK 23+
• AGP 8.9.1 is the latest stable compatible with Gradle 8.12
• AGP 8.x requires namespace in build.gradle (removed from AndroidManifest.xml)

Test plan

• Ran dart tool/ci_script.dart — all packages pass (2 pre-existing failures unrelated to this change: compass_app/server and cupertino_gallery)
• Verified no formatting regressions (dart format clean)
• Verified no analysis issues (dart analyze clean)

Fixes #2568