public boolean isSystemSignature(final PackageInfo pkg) {
		if (sSystemSignature == null) {
			sSystemSignature = new Signature[] { getSystemSignature(mContext.getPackageManager()) };
		}
		return sSystemSignature[0] != null && sSystemSignature[0].equals(getFirstSignature(pkg));
	}
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