Instruct users to run Android Studio so it will install Android SDK components (#9323)

Fixes https://github.com/flutter/flutter/issues/8690

packages/flutter_tools/lib/src/android/android_workflow.dart

@@ -103,7 +103,8 @@ class AndroidWorkflow extends DoctorValidator implements Workflow {
         ));
       } else {
         messages.add(new ValidationMessage.error(
-          'Android SDK not found. Download from https://developer.android.com/sdk/\n'
+          'Install Android Studio from https://developer.android.com/studio/index.html.\n'
+          'On first launch it will assist you in installing the Android SDK components.\n'
           '(or visit https://flutter.io/setup/#android-setup for detailed instructions).'
         ));
       }