Improve the logic to compile for Android by automatically detecting and setting up the Android NDK if needed.

platform/android/java/app/build.gradle

@@ -98,6 +98,8 @@ android {
         disable 'MissingTranslation', 'UnusedResources'
     }
 
+    ndkVersion versions.ndkVersion
+
     packagingOptions {
         exclude 'META-INF/LICENSE'
         exclude 'META-INF/NOTICE'

platform/android/java/app/config.gradle

@@ -7,7 +7,8 @@ ext.versions = [
     supportCoreUtils   : '1.0.0',
     kotlinVersion      : '1.4.10',
     v4Support          : '1.0.0',
-    javaVersion        : 1.8
+    javaVersion        : 1.8,
+    ndkVersion         : '21.3.6528147' // Also update 'platform/android/detect.py#get_project_ndk_version()' when this is updated.
 
 ]