Make mismatched signatures for system APKs using shared users fatal

The previous behavior here just skipped installing whichever package(s)
happened to be scanned later, which is race-y, and otherwise was silent.
Promote this to blowing up the system instead so it's very obvious that
something is quite broken, and let Rescue Party handle it.

(From review comments on ag/3967497)

Bug: 74501739
Test: Created release-keys build locally but treated one package
(com.android.mtp) as PRESIGNED so it kept dev-keys.

Change-Id: Ibd04ec988afdf4b57087e20795c8fa5ab5d52e37