Resolve IndexOOBException in SigningDetails#hasAncestor

When a package that is part of a shareduid is installed / updated its
lineage is compared against the current signature / lineage for the
shareduid. If the new package has more than one signature in the lineage
an IndexOutOfBoundsException is thrown by SigningDetails#hasAncestor
crashing the system server. This commit ensures the ancestor check
is only performed against the current signer for the shareduid.

Fixes: 155134046
Test: atest SigningDetailsTest
Change-Id: Icef9f21c6901e255e5276085259d2f773f41e858