The Package Manager handles this now. To share a UID, all packages
must be signed with exactly the same set of certs.
Change-Id: I2fd08923f55f02ae2f1d503266ab124be2472921
Setting LOCAL_CERTIFICATE to "EXTERNAL" now marks an apk (either a
prebuilt or otherwise) as needing the default test key within the
system, but one that should be signed after the target_files is
produced but before sign_target_files_apks does the rest of the
signing. (We use this to ship apps on the system that are signed by
third parties, like Facebook.)
The check_target_files_signatures determines what key was used to sign
every .apk in a given target_files. It can compare that signature to
that of another target_files (eg, the previous release for that
device) and flag any problems such as .apks signed with a different
key.
