Added hasSha256SigningCertificate to the PacakgeManagerService implementation.

The hasSha256SigningCertificate function was added to the PM aidl interface.
This change implements it by connecting it to the existing
PackageManager function.

Bug: 174605881
Test: verified the PM service to work properly on a local device.
Change-Id: Ia7eb1a371778c78aa201199e43b7c5719722113f