Fix incorrect APK file type

The previous versions of mirror_ball-Vulkan-release.apk turned out to be the wrong type of APK file to upload. So I've uploaded what I hope is the correct one. I've installed it using Sidequest on both the original Quest model and a Quest 2 and both function as expected. My apologies to those of you who attempted to install the old one. I'm still getting the hang of this Android stuff.

Note to other Android developers: The APKs you find down build/intermediates/apk are NOT the right ones to install with Sidequest, they only work with the Android Studio debugger. You want the ones down build/outputs/apk, which are produced by running Build->Build Bundle(s)/APK(s)->Build APK(s).