Explicitly set ndkVersion to Flutter's (#247)

Without this, webcrypto.dart uses AGP's default NDK version, which
can be different from Flutter's preferred NDK version. Apart from
causing builds to download multiple NDKs needlessly, this also
causes APKs to not use 16 KB page sizes.

Tested by building an APK and opening it in Android Studio's APK
analyzer.

Author: emersion

android/build.gradle

@@ -28,6 +28,7 @@ android {
     namespace 'dev.google.webcrypto'
 
     compileSdk = flutter.compileSdkVersion
+    ndkVersion = flutter.ndkVersion
 
     // We depend on cmake for the native parts.
     externalNativeBuild {