Android: Module builds should auto-download NDK r21c by default if needed

Summary:
When doing a module build, Titanium will auto-download NDK r20b (aka: 20.1.5948944) by default if NDK installations were found on the system.

The default should be updated to NDK r21c (aka: 21.2.6472646).
https://github.com/android/ndk/wiki/Changelog-r21#r21c

Reason:
The macOS distribution of the Android NDK r21c tools have been "notarized" by Google. This means module builds will no longer display several security popups on macOS Catalina.

To-Do:
Update the android.ndkVersion here...
https://github.com/appcelerator/titanium_mobile/blob/master/android/templates/module/generated/build.gradle