Support for an 'end session' protocol message.

1) On the push side, this message is a flag in PushMessageContent.
   Any secure message with that flag will terminate the current
   sessin.

2) On the SMS side, there is an "end session" wire type and
   the convention that a message with this wire type must be
   secure and contain the string "TERMINATE."

src/org/thoughtcrime/securesms/ReceiveKeyActivity.java

@@ -236,7 +236,7 @@ public class ReceiveKeyActivity extends Activity {
 
               DecryptingQueue.scheduleDecryption(ReceiveKeyActivity.this, masterSecret, messageId,
                                                  threadId, recipient.getNumber(), recipientDeviceId,
-                                                 messageBody, true, false);
+                                                 messageBody, true, false, false);
             } catch (InvalidKeyIdException e) {
               Log.w("ReceiveKeyActivity", e);
               DatabaseFactory.getEncryptingSmsDatabase(ReceiveKeyActivity.this)