Preprovision Android NDK for flavored builds and reuse matching unflavored NDKs (#183555)

Flutter currently forces AGP to download the Android NDK by configuring
a
synthetic empty CMake project in `forceNdkDownload()`.

For flavored Android apps, that synthetic native configuration can cause
AGP
to fold generic CMake tasks across flavors. In practice, building one
flavor
can pull native/CMake work from another flavor into the active build.

This change moves NDK provisioning into the Flutter tool flow for
flavored app
builds:
- query the effective `ndkVersion` with a lightweight Gradle metadata
task
- install the required NDK before the main build when needed
- pass a preprovisioned NDK property so `forceNdkDownload()` can return
early

To avoid adding unnecessary Gradle overhead to common unflavored builds,
the
Flutter tool now also skips `forceNdkDownload()` when an existing NDK is
already discoverable from the Android SDK or standard Android NDK
environment
variables.

This preserves the original goal of ensuring the NDK is available, while
avoiding flavored CMake/native graph contamination.

Manual validation:
- verified on a minimal flavored Android app that `dev` builds no longer
pull
  `prod` native/CMake work into the task graph
- verified locally on a downstream flavored Flutter app for `build apk`,
  `build appbundle`, and `flutter run`

Fixes #183296

---------

Co-authored-by: Gray Mackall <34871572+gmackall@users.noreply.github.com>