feat: modernize Android build toolchain

[jokerttu]

Ports the Android build modernization from flutter-navigation-sdk#711 to the driver SDK.

• Convert the plugin's android/build.gradle + settings.gradle to Kotlin DSL (.kts).
• Plugin: apply KGP conditionally (only on AGP < 9). AGP 9+ ships built-in Kotlin, so applying KGP there is unnecessary and triggers a Flutter deprecation warning.
• Replace deprecated kotlinOptions {} with kotlin { compilerOptions {} } in plugin and example.
• Raise Java/Kotlin target 11 → 17 (matches Flutter templates and Navigation SDK requirement).
• Bump example Gradle wrapper 8.13 → 8.14.3 and Kotlin (KGP) 2.1.0 → 2.3.20.
• Gate the ktfmt formatter behind -Pktfmt (via melos run format:android) so it is never forced onto apps that depend on this plugin.

Driver-specific dependencies (transportation-driver:7.0.0, androidx.startup) are preserved; the navigation SDK's play-services-maps exclusion is intentionally not ported since the driver SDK never had it.

Verification

• flutter build apk --config-only succeeds.
• ./gradlew :google_driver_flutter:compileDebugKotlin builds on Gradle 8.14.3 / JVM target 17.
• ./gradlew :google_driver_flutter:testDebugUnitTest passes (mockk + robolectric load under JVM 17).
• Confirmed transportation-driver:7.0.0 resolves and pulls in navigation:7.6.0 transitively.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I updated/added relevant documentation
• I added new tests to check the change I am making
• All existing and new tests are passing.

🤖 Generated with Claude Code

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