Migrate to new AGP DSL and remove android.newDsl=false opt-out

[runningcode]

Follow-up to the AGP 8.13.1 → 9.2.1 bump (#5777). To land AGP 9 without a large refactor, the bump added a temporary opt-out in gradle.properties:

# AGP 9+ migration opt-outs until we remove kotlin-android plugin and adopt built-in Kotlin.
android.newDsl=false

Why this matters: android.newDsl=false opts out of AGP 9's new DSL. If Google removes this flag in a future AGP release, we will be unable to upgrade until the migration is done.

Current state:

• The root build.gradle.kts (around lines 174-180) still configures library modules via the legacy com.android.build.gradle.BaseExtension inside plugins.withId("com.android.library") { ... }. This is what depends on the old DSL.
• Runtime modules already use the new androidComponents Variant API (beforeVariants/onVariants); the old libraryVariants/applicationVariants/BaseVariant API is not used anywhere. Good starting position.
• Fragility to address: sentry-samples/sentry-samples-android/build.gradle.kts imports the internal com.android.build.api.variant.impl.VariantImpl (a non-stable .impl. class), a likely future break point.

Scope:

• Replace the BaseExtension-based configuration in the root build with the new DSL.
• Remove the internal VariantImpl usage in the Android sample.
• Remove android.newDsl=false from gradle.properties and confirm the build passes.

Done when: android.newDsl=false is removed and the build works on the new AGP DSL.

Should be picked up after the AGP bump PRs merge.

[linear-code]

JAVA-650