Align Android build logic with AGP 9

[cdsap]

Summary

• omit the Kotlin Android plugin from generated AGP 9 Android root builds so built-in Kotlin is used
• configure app/library Java compatibility and toolchains through Android compileOptions plus JavaPluginExtension
• use com.android.legacy-kapt for AGP 9 KAPT fallback and keep KSP as the default path
• make Android E2E tests write local.properties from a detected SDK or skip clearly when no SDK is configured

Verification

• ./gradlew :project-generator:test --tests io.github.cdsap.projectgenerator.generator.rootproject.BuildGradleTest --tests io.github.cdsap.projectgenerator.writer.ConventionPluginWriterTest --tests io.github.cdsap.projectgenerator.generator.buildfiles.BuildFilesGeneratorAndroidTest --tests io.github.cdsap.projectgenerator.RoomDiVariantsAssembleE2EValidationTest --tests io.github.cdsap.projectgenerator.AndroidKotlinMultiplatformLibraryE2EValidationTest
• Result: BUILD SUCCESSFUL

Note: Develocity scan publishing reports missing authentication locally, but it does not affect the test result.