Gallery2: fix display abnormal when select all on timeline page.

1. number of selection display more than actual selection,
because it include title items.
calculate correct number of selection exclude title items.
2. share and delete icon not display because an Exception occurs.
Override getContentUri() method in MediaSet.java to avoide exception
because getContentUri() of MediaObject.java must be override.

Change-Id: Ida549d6f7f5ebfea344dcdfd47f7dad48ba1f65d
CRs-Fixed: 948409