PackageManager: Add configuration to specify vendor platform signatures

Devices with split system/vendor images may want to use the OEM's vendor
image. In that case, the OEM's platform signature is not actually the
same as the platform signature used to sign the Lineage system image.
Allow devices to specify an OEM platform signature which will also be
recognized as the system's platform signature.

Change-Id: Ida9bb25a32234af9d9507a214eae6a4672320d2b