Reland: pm: Scan-as-privileged apps that share a privileged user

Commits 2ede0473 and
77029c5b touch the same code in
PackageManager. They landed in master at the same time causing
a build breakage. Reland with fixes to PackageSetting signatures
usage.

Bug: 72235911
Test: build/flash Taimen, verify userdictionary provider
    is scanned as priv-app.
Test: gts-tradefed run everything

Change-Id: I57818409fa2f21c4f8f54651dd539790005d3e92