Add knownCerts to system API and copy constructor.

PermissionInfo.PROTECTION_FLAG_KNOWN_SIGNER is already @SystemApi, and
we need system API for accessing the actual certs in the modularized
permission subsystem.

The copy constructor missed knownCerts in ag/13504004 and is fixed now.
knownCerts isn't deep copied in the copy constructor because
PackageItemInfo.metaData isn't deep copied either.

Bug: 182523293
Test: atest CtsContentTestCases:PermissionInfoTest
Change-Id: I784764fdf7238f6b10455ff40c693de0823a94cb