StorageManager: only register MountService listener when needed

When other service get StorageManager, StorageManager will register a
listener in MountService, which won't be unregistered.

So change the policy to be:
1. when other service needs to register listener in StorageManager,
StorageManager will register listener in MountService.
2. When other service needs to unreg0ster listener in StorageManager,
if there is no more other listeners in StorageManager, StorageManager
will unregister listener in MountService

Change-Id: Ie59ad96279ac3b51f7a606fcfbbde0c130cf61dc
Author: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Bing Deng <bingx.deng@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: 30943