Revert "Bump NDK version to match version available in ci"

This reverts commit 849c2fd47d.

playground/android/app/build.gradle

 
 android {
     compileSdkVersion 28
-    ndkVersion "21.1.6352462"
+    ndkVersion "20.1.5948944"
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8