Make microG signature spoofing not rely on priv-app

Summary

We initially did this as a security measure, 2 better than 1, but we can achieve the same via other ways

Details

PoC: 17249: microG: change perm to 'signature' | https://review.calyxos.org/c/CalyxOS/platform_frameworks_base/+/17249

Will need a stub app somewhere (put it anywhere for now), all that does is have the signature, it doesn't need to do anything else at all.