I have written a small test app on nativescript playground online, and I want .apk file of it. Is there any online tool to generate .apk file from playground, instead of having to install tns and run 'tns run'/'tns debug'/'tns build' on my local machine ?
No, it's not possible to generate APK from Playground.
If you don't want to setup Android SDK locally, you may try cloud build option but at least that requires NativeScript CLI / Sidekick setup on your local machine.