Unify add-to-app module wiring on the variant API and drop host-project cross-wiring

[reidbaker-agent]

Phase 8 of 11 of the AGP public-API migration; stacked on #7.

Phase P7 of the AGP public-API migration (flutter#180137,
flutter#166550), the last Kotlin consumer of the legacy variant
API:

• Add-to-app module (library) projects now use the same consolidated
  onVariants block as application projects: the Flutter compile task
  and CopyFlutterAssetsTask are registered lazily per library variant,
  and flutter_assets are wired through
  variant.sources.assets.addGeneratedSourceDirectory. The host
  application consumes them through AGP's normal library packaging and
  variant matching, with build modes resolved from the public
  Component.debuggable flag (so a custom debuggable host build type
  such as 'staging' still maps to the debug engine artifacts via the
  module's matched variant). Module variants skip the CLI task-name
  gating: which module variant a host build consumes is AGP's variant
  matching decision, and a module has at most three variants to
  configure lazily.
• The host-project lookup, the appProject.afterEvaluate
  libraryVariants.all x applicationVariants.all cross-product, the
  explicit merge<HostVariant>Assets.dependsOn edge, and the
  addFlutterDepsForModule legacy fork from P5 are deleted.
• flutter.hostAppProjectName is now a no-op: it only fed the host
  lookup. A warning explains that it has no effect and will be removed
  in a future Flutter release.
• The legacy com.android.builder.model.BuildType buildModeFor overload
  lost its last caller and is deleted along with its duplicate tests
  (the P2 name/debuggable and DSL overload tests cover the semantics).
  FlutterPlugin.kt and FlutterPluginUtils.kt no longer import any
  legacy (non com.android.build.api) AGP types.

Verification (CI): build_android_host_app_with_module_source,
module_host_with_custom_build, module_custom_host_app_name scenarios;
module-root flutter build apk; add-to-app newDsl=true test; gradle unit
tests on both AGP axes.

Revert-safe until P10 lands (mutually tolerant with P6).

CI must run the FGP unit tests (the Kotlin suite in packages/flutter_tools/gradle could not run in the delivery sandbox: dl.google.com returns 403, so AGP artifacts do not resolve).