PackageSignatures: bring back tag skipping on unknown element.

Commit: 7ee17aa6 moved the
skipCurrentTag call when reading certs into the branch directly
dealing with <cert /> tags to eliminate issues for the <pastSigs />
tag, but in doing so will cause bad parses when encountering unknown
elements.  Restore the previous behavior when an unknown tag is
encountered.

Bug: 64686581
Test: Builds, boots.
Change-Id: I3e2850b3dc1660ca43b7b5d01bf345290d11bac0