Preserve original encoded certificate bytes.

Sadly, the publicly visible android.content.pm.Signature object needs
to return the untouched bytes from the parsed APK certificate.  Since
some CertificateFactory implementations may try re-encoding the
certificate, we defensively keep around the original encoded bytes
and return them when asked.

(cherry-pick of commit 0c990ab4a90b8a5492a67b2b728ac9a4a1ccfa1b)

Bug: 18206852, 18228011
Change-Id: I6016eaea23a7f43cf9249ec43cfde1f8ed4ff841