Don't issue a deletion for the global metadata backup

We were accidentally submitting a deletion for the global metadata key in the
PM backup handling (it was falling into the usual "here's a package that we said
we'd backed up last time, but now it's no longer on device" code).  Don't do
that any more, i.e. actually keep the global metadata key in the backup set.
Oops.

services/java/com/android/server/PackageManagerBackupAgent.java

@@ -120,7 +120,10 @@ public class PackageManagerBackupAgent extends BackupAgent {
             // write its signature block to the output, keyed on the package name.
             for (PackageInfo pkg : mAllPackages) {
                 String packName = pkg.packageName;
-                if (!existing.contains(packName)) {
+                if (packName.equals(GLOBAL_METADATA_KEY)) {
+                    // We've already handled the metadata key; skip it here
+                    continue;
+                } else if (!existing.contains(packName)) {
                     // We haven't stored this app's signatures yet, so we do that now
                     try {
                         PackageInfo info = mPackageManager.getPackageInfo(packName,