Remember which apps have available restore info in the ancestral dataset

When we perform a full-system restore, remember the set of applications which
have data available in our ancestral dataset.  This is a key filter for not
having to do a round trip to the [remote] storage backend at app-install time
unless it is likely to be fruitful.

Change-Id: I7c77b490c560c581888d84f02f258b2e2d73bc69

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

@@ -40,6 +40,7 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 /**
  * We back up the signatures of each package so that during a system restore,
@@ -100,6 +101,20 @@ public class PackageManagerBackupAgent extends BackupAgent {
 
         return mRestoredSignatures.get(packageName);
     }
+
+    public Set<String> getRestoredPackages() {
+        if (mRestoredSignatures == null) {
+            Log.w(TAG, "getRestoredPackages() before metadata read!");
+            return null;
+        }
+
+        // This is technically the set of packages on the originating handset
+        // that had backup agents at all, not limited to the set of packages
+        // that had actually contributed a restore dataset, but it's a
+        // close enough approximation for our purposes and does not require any
+        // additional involvement by the transport to obtain.
+        return mRestoredSignatures.keySet();
+    }
     
     // The backed up data is the signature block for each app, keyed by
     // the package name.