Compile the Flutter Gradle Plugin against the gradle-api artifact only

Phase P10, the final phase of the AGP public-API migration
(flutter/flutter#180137, flutter/flutter#166550):

- build.gradle.kts swaps both AGP dependencies from the full
  com.android.tools.build:gradle artifact to
  com.android.tools.build:gradle-api. Compilation is now the proof that
  the plugin uses zero AGP internals (AGP 10 removes access to them
  entirely), on top of the InternalAgpApiImportTest source check and
  the validateNoCommonExtensionInBytecode bytecode check, which stay as
  fast-feedback guards.
- FlutterPluginTest is rewritten on public-API mocks only: the
  AbstractAppExtension multi-interface mock, BaseExtension,
  CommonExtension, internal.dsl.DefaultConfig, and the legacy
  sourceSets/AndroidSourceDirectorySet mocks are gone. No test source
  references a full-artifact-only or internal type anymore.
- The --android-skip-build-dependency-validation help text now warns
  that skipping validation cannot make unsupported AGP versions work:
  builds below the supported minimum fail in Gradle regardless.
- The android tooling README gains the AGP-bump checklist step to run
  the FGP test suite against the new version's public API artifact
  (-PagpVersion axis) and links the migration doc.

Verification (CI): full matrix per the migration doc (gradle unit tests
on both AGP axes - now both against gradle-api; scratch-app matrix;
add-to-app source/AAR flows; config-cache baseline). Dart analyzer
clean on the changed tool file.

Cleanly revertible in isolation.