Add imgdiag support for diff against zygote

Adds option --zygote-diff-pid. If this option is passed in then it
tries to differentiate objects that are private dirty by comparing
them against the zygote image contents if they are different from
the boot image contents.

Bug: 27906566

Change-Id: I6d214f73af5bf6de1f1a07f309065fc455cf7543