Fix null pointer exception during initialization

PackageManagerService ctor calls
InitAndSystemPackageHelper.initPackages() to create the
OverlayConfig object, but this object is needed inside the
initPackages() call, when parsing an updated system overlay
in ScanPackageHelper.assertPackageIsValid() -> isOverlayMutable

This change uses the OverlayConfig's singleton accessor as a
backup when the member isn't set

Bug: 211469178
Test: install APK from the bug, reboot - no crashloop
Change-Id: I0d2b2fc1317db963980dbd1ca84dc61d2bfec5f5