[native_assets_cli][c_compiler] Android NDK version

[dcharkes]

We need the launcher (Flutter) to specify the minimum Android API level when compiling, otherwise we can get an API mismatch such as:

           Invalid argument(s): Couldn't resolve native function 'sum' in
           'package:my_package/my_package_bindings_generated.dart' : Failed to load dynamic library
           'libmy_package.so': Failed to load dynamic library 'libmy_package.so': dlopen failed: cannot locate
           symbol "pthread_atfork" referenced by
           "/data/app/com.example.my_package_example-2/lib/arm64/libmy_package.so"....

https://android.googlesource.com/platform/bionic/+/master/android-changes-for-ndk-developers.md

In the Flutter FFI plugins this is currently done by passing it to the build system. And it's checked to be compatible between final app and plugins.

In the CMake builds prototype I had it hardcoded.

We should let Flutter populate a field in the Config so that we can read that in package:c_compiler. (must have)

In addition, a package might want to specify an NDK lower bound as well. We could try to standardize that in the pubspec.yaml and read it in into the config, or have the package override the value in their build.dart. We can't rely on Flutter infra for this, because we don't have the plugin.gradle file anymore (none of the boilerplate files) with native assets. (nice to have, probably not needed for MVP)

I don't believe we have to concern ourselves with the end user specifying an NDK version. If the end-user does that, it should be propagated by Flutter and be passed from Flutter to the config instead of from the end user directly to us. So we don't have a need for #39 here.