Adjust NDK version

Author: bailuk

app/build.gradle

@@ -3,7 +3,10 @@ apply plugin: 'com.android.application'
 android {
     compileSdkVersion 28
 
-    ndkVersion "21.1.6352462"
+    /* Set this to whatever version is installed on the GitHub build environment
+    and hope it works everywhere.
+    ndkVersion "21.1.6352462" */
+    ndkVersion "21.3.6528147"
 
     defaultConfig {
         applicationId "ch.bailu.aat"