Rearrange decrypt API.

1) Change SessionBuilder to only establish sessions via
   KeyExchangeMessage and PreKeyBundles.

2) Change SessionCipher to decrypt either WhisperMessage
   or PreKeyWhisperMessage items, automatically building
   a session for the latter.

3) Change SessionCipher to tear down new sessions built
   with PreKeyWhisperMessages if the embedded WhsiperMessage
   fails to decrypt.

library/src/org/whispersystems/textsecure/crypto/SessionCipherFactory.java

@@ -1,41 +0,0 @@
-/** 
- * Copyright (C) 2013 Open Whisper Systems
- * 
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- * 
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
-package org.whispersystems.textsecure.crypto;
-
-
-import android.content.Context;
-
-import org.whispersystems.libaxolotl.SessionCipher;
-import org.whispersystems.libaxolotl.state.SessionStore;
-import org.whispersystems.textsecure.storage.RecipientDevice;
-import org.whispersystems.textsecure.storage.TextSecureSessionStore;
-
-public class SessionCipherFactory {
-
-  public static SessionCipher getInstance(Context context,
-                                          MasterSecret masterSecret,
-                                          RecipientDevice recipient)
-  {
-    SessionStore sessionStore = new TextSecureSessionStore(context, masterSecret);
-
-    if (sessionStore.containsSession(recipient.getRecipientId(), recipient.getDeviceId())) {
-      return new SessionCipher(sessionStore, recipient.getRecipientId(), recipient.getDeviceId());
-    } else {
-      throw new AssertionError("Attempt to initialize cipher for non-existing session.");
-    }
-  }
-}