Settings: show low storage notify only for internal storage

low memory notification indecates the storage status for internal
storage. So before show this notification, check whether it is internal
storage

Change-Id: Ibba5cefde360a073ac470cdb42bbcc87be81b78d
Author: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 62433