build: migrate project to AGP 9

[t-regbs]

Summary

• upgrade to AGP 9.3.1 and Gradle 9.6.1, adopt built-in Kotlin, and assign unique Android KMP namespaces
• update Kotlin, KSP, Firebase, Google Services, and Gradle utility plugins for AGP 9 compatibility
• keep Compose Multiplatform aligned at 1.10.3 to preserve Calf/Compottie native binary compatibility
• fix Android Compose test dependency resolution and make the AlarmKit-only iOS test independent of an app bundle

Verification

• ./gradlew build
• ./gradlew :androidApp:assembleDebug :androidApp:assembleRelease
• ./gradlew :shared:linkDebugFrameworkIosSimulatorArm64 :shared:linkReleaseFrameworkIosSimulatorArm64 :shared:linkReleaseFrameworkIosArm64 --rerun-tasks
• confirmed no Calf/Compottie Kotlin/Native partial-linkage diagnostics in final framework links