Make the AdGuard application update work on Android 11

Well, we have to implement our own Download Manager to use it on Android 11 to download AdGuard updates. If we don't, users won't be able to update later. :(
See more about the Google Download Manager implementation [here](https://github.com/AdguardTeam/AdguardForAndroid/issues/3516#issuecomment-678241177).