Add warning log for mismatched signatures

It's difficult to see in bugreports when this situation arises. Add a
small log so we can easily determine installation failure reason.

Change-Id: Ie59c205cf731cad7b3d04ceb995e58a093c62455

Author: kruton

services/java/com/android/server/pm/PackageManagerService.java

@@ -6614,6 +6614,7 @@ private void replacePackageLI(PackageParser.Package pkg,
             oldPackage = mPackages.get(pkgName);
             if (compareSignatures(oldPackage.mSignatures, pkg.mSignatures)
                     != PackageManager.SIGNATURE_MATCH) {
+                Slog.w(TAG, "New package has a different signature: " + pkgName);
                 res.returnCode = PackageManager.INSTALL_PARSE_FAILED_INCONSISTENT_CERTIFICATES;
                 return;
             }