Prepare for: disallow installation of non-preloads using system certs or uids.

This does not actuall change anything, just a preparation.
The subsequent much smaller CL will disable such installs in
non-debuggable builds.

Bug: 308573259
Test: presubmit
Change-Id: I95bdbc20a8c5be5426d30d0b19d4b4cb82769ed9