Install NDK r21 on GitHub Actions (#3961)
Install NDK r21 on GitHub Actions (#3961)

NDK r21 is no longer provided by default on GitHub Actions.

https://github.com/actions/virtual-environments/issues/5930
firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk