Improve F-Droid Filter

Hi,

I was looking at the code for the F-Droid filter and noticed that it checks the cert info of the app.

An additional check could be done for the installerPackageName of the app.

https://developer.android.com/reference/android/content/pm/PackageManager#getInstallerPackageName(java.lang.String)

This value would be set to org.fdroid.fdroid.privileged if that is installed, so it should cover all F-Droid apps for those users.