Fix logging-out all accounts in sample app

- clearing the adapter needs to be done on the UI thread

box-content-sample/src/main/java/com/box/androidsdk/sample/MainActivity.java

@@ -81,7 +81,7 @@ private void configureClient() {
      * Create a BoxSession and authenticate.
      */
     private void initSession() {
-        mAdapter.clear();
+        clearAdapter();
         mSession = new BoxSession(this);
         mSession.setSessionAuthListener(this);
         mSession.authenticate(this);