private static Signature getFirstSignature(final PackageInfo pkg) {
		if (pkg != null && pkg.signatures != null && pkg.signatures.length > 0)
			return pkg.signatures[0];
		return null;
	}
Determine whether a package is a "system package", in which case certain things
(like disabling notifications or disabling the package altogether) should be disallowed.
shared/src/main/java/com/oasisfeng/island/provisioning/SystemAppsManager.java