use chooser for sendmail (remove explicit gmail) in LogHistory

Fixing issue https://github.com/owncloud/android/issues/689#issuecomment-60972684

Continue there for discussion.
Please, replace ```ownCloud``` with a parameter marker ```%1$s```. Then use ```String.format(...)``` in code to complete it with the value of ```getString(R.strings.app_name)```.

You may have a look at .ui.activity.Preferences.java#L259, as a reference.