Add PackageSignatures readXml tests

This fix also refactors PackageParser.SigningDetails to move the
pastSigningCertificatesFlags to be a data member of Signature; this
allows the capabilities of a previous signing certificate to be
accessed directly from the Signature object as opposed to relying
on the 1-1 mapping of the past certs and flags in the SigningDetails.

Fixes: 73927696
Fixes: 73925989
Test: adb shell am instrument -w -e class com.android.server.pm.PackageSignaturesTest \
      com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: I635f2d2209350d066d1fa2ef07460071da0c023e