Set installer package name to privext if using that

Chirayu Desairequested to merge
cde/fdroidclient:uninstall-v0.102.2 into stable-v0.102
  • The new PackageInstaller APIs, being used by the privext on Android 7.0 and above aren't happy with uninstall being done by an app other than the original installer.
  • Set it to the privileged extension if that is enabled and being used, to make uninstalling work

Fixes privileged-extension#23 (closed) Same as !454 (merged) but for stable (as the code is slightly different)

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