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/database/EncryptingSmsDatabase.java

@@ -73,7 +73,7 @@ public class EncryptingSmsDatabase extends SmsDatabase {
   {
     long type = Types.BASE_INBOX_TYPE;
 
-    if (!message.isSecureMessage()) {
+    if (!message.isSecureMessage() && !message.isEndSession()) {
       type |= Types.ENCRYPTION_SYMMETRIC_BIT;
       message = message.withMessageBody(getEncryptedBody(masterSecret, message.getMessageBody()));
     }