FileDisplayActivity: wait until OCFileListFragment is initialized to trigger browseToRoot()

This solves a bug which caused a "Directory does not exist" popup when coming back from Gallery,
as it tried to call browseToRoot on GalleryFragment due it being executed before the fragment transaction.

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

app/src/main/java/com/owncloud/android/ui/activity/FileDisplayActivity.java

@@ -513,6 +513,7 @@ public class FileDisplayActivity extends FileActivity
                 Log_OC.d(this, "Switch to oc file fragment");
 
                 setLeftFragment(new OCFileListFragment());
+                getSupportFragmentManager().executePendingTransactions();
                 browseToRoot();
             }
     }