Go back to support for Android API 16+

[pclemins]

With the update to Qt 5.14.1, the new Android minimum API is 21. We have the compiled SSL for 16+, we just need to figure out how to include an APK for 16-20. Looks like Qt's older version for support is 5.9.x and I think we can probably do that...

Maybe moving to a bundle? https://developer.android.com/platform/technology/app-bundle
Looks like Qt has support for that.

Multiple APKs: https://developer.android.com/google/play/publishing/multiple-apks

[pclemins]

Oh... and... we definitely want to separate apks because Android 10 now prompts users if they try to install an APK targetting API 22 or lower.

[pclemins]

We may need NDK older than r19... one that still has g++ in the toolchain (<r18b) because Qt 5.9.9, defaults to g++ and I couldn't seem to change it in Qt Composer. We might try Qt 12.x...

One can switch NDK version from the default latest downloaded one with android.ndkVersion property in build.gradle (https://developer.android.com/studio/releases/gradle-plugin).

Or.... write our own build script.