Update Android CI to support API level 31

In !2514 (merged), we dropped support for building API level 21 in CI, as it’s for a no-longer-supported version of Android. That leaves us building API level 28, which is for Android 9. Android 9 is currently the oldest supported version of Android. The latest supported version is 12 (API level 31).

We should be testing against the newest and oldest supported API versions, so we need to add CI support for testing against version 31.

This will require rebuilding the Android CI image (android-ndk.Dockerfile). It should be updated to a supported version of Fedora at the same time (currently it’s using Fedora 33)