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.
(This is a backport of a change in froyo. It does not affect device
code, only the scripts that sign images and OTA packages.)
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