generate public keyring from secret if not available

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/pgp/UncachedKeyRing.java

@@ -27,6 +27,7 @@ import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.Date;
 import java.util.HashSet;
@@ -759,6 +760,21 @@ public class UncachedKeyRing {
 
     }
 
+    public UncachedKeyRing extractPublicKeyRing() {
+        if(!isSecret()) {
+            throw new RuntimeException("Tried to extract public keyring from non-secret keyring. " +
+                    "This is a programming error and should never happen!");
+        }
+
+        ArrayList<PGPPublicKey> keys = new ArrayList();
+        Iterator<PGPPublicKey> it = mRing.getPublicKeys();
+        while (it.hasNext()) {
+            keys.add(it.next());
+        }
+
+        return new UncachedKeyRing(new PGPPublicKeyRing(keys));
+    }
+
     /** This method replaces a public key in a keyring.
      *
      * This method essentially wraps PGP*KeyRing.insertPublicKey, where the keyring may be of either