title: UnitySendMessage not working from non-unity activity
body: <p>I'm developing an android plugin which should start another activity (with UI and everything...).
  This activity (which is not inherits from UnityPlayerActivity of course) should send message to the Unity C# code.
  I'm trying to <strong>call UnityPlayer.UnitySendMessage</strong>, I see my log a moment before sending the message but the <strong>C# side doesn't get it</strong> (no exception is raised).</p>
  
  <p><em>This is the callback which is called from the other activity:</em></p>
  
  <pre><code>@Override
  public void generatePayload() {
     try {
        Log.v(TAG, "generatePayload was triggered");
        UnityPlayer.UnitySendMessage("AndroidObject", "generatePayloadMessage", "");
     } catch (Exception ex) {
        Log.e(TAG, "failed to send message to unity");
     }
  }
  </code></pre>
  
  <ul>
  <li>I've tried to call this method from the unity primary activity and the message was received successfully in the unity side.</li>
  <li>I've tried also to send the message from the other activity (as I should) but in main thread and it didn't work too...</li>
  </ul>
  
  <p>Any suggestions? Can't I send message to unity while UnityActivity isn't in foreground? If this is the situation - what can I do?</p>