Revert "[deps] Stop moving NDK folder inside Android SDK (#47454)" (flutter/engine#47905)

Reverts

* https://github.com/flutter/engine/pull/47454

and adds documentation

The NDK folder shuffling in the DEPS is to simplify the build files and enable building with different NDK versions between Dart and Flutter while sharing the build files.

Closes: https://github.com/flutter/flutter/issues/136666

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/tools/android_sdk/create_cipd_packages.sh

@@ -115,6 +115,17 @@ for platform in "${platforms[@]}"; do
     done
   done
 
+  # Special treatment for NDK to move to expected directory.
+  # Instead of the ndk being in `sdk/ndk/<major>.<minor>.<patch>/`, it will be
+  # in `ndk/`.
+  # This simplifies the build scripts, and enables version difference between
+  # the Dart and Flutter build while reusing the same build rules.
+  mv $upload_dir/sdk/ndk $upload_dir/ndk-bundle
+  ndk_sub_paths=`find $upload_dir/ndk-bundle -maxdepth 1 -type d`
+  ndk_sub_paths_arr=($ndk_sub_paths)
+  mv ${ndk_sub_paths_arr[1]} $upload_dir/ndk
+  rm -rf $upload_dir/ndk-bundle
+
   # Accept all licenses to ensure they are generated and uploaded.
   yes "y" | $sdkmanager_path --licenses --sdk_root=$sdk_root
   cp -r "$sdk_root/licenses" "$upload_dir/sdk"