fix build error to upgrade NDK

Author: jeesang7

android/app/build.gradle

@@ -26,7 +26,7 @@ apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
     compileSdkVersion 28
-    ndkVersion "21.4.7075529"
+    ndkVersion "26.0.10792818"
 
     lintOptions {
         disable 'InvalidPackage'