lib.systems: throw if sdkVer or ndkVer are used for android.

Those attrs have been renamed and throwing is the best way to show it,
if we only warned then the user would only get an error like this `error: Unsupported sdk: 33`
from `pkgs/top-level/darwin-packages.nix`.

If someone wants to support multiple NixOS versions then they can simply
set both attrs. (`!args ? androidSdkVersion` is for that)

lib/systems/examples.nix

@@ -60,6 +60,8 @@ rec {
   armv7a-android-prebuilt = {
     config = "armv7a-unknown-linux-androideabi";
     rust.rustcTarget = "armv7-linux-androideabi";
+    androidSdkVersion = "33";
+    androidNdkVersion = "26";
     useAndroidPrebuilt = true;
   } // platforms.armv7a-android;