UsbSettings: Adjust UMS mode code logic.

Not to update UMS status in every iteration of client listener.

Change-Id: I5fbc0145c798c468f2ae943b251700f1a71e00d8
Make sure emulated storage path index is 0

MediaProvider blindly assumes that path index 0 of the array returned by
StorageManager::getVolumePaths() is the emulated storage.
However, this is not true when you have multiple users.
This patch modifies StorageVolume::getVolumeList(). to make sure that
assumption is always true.

Change-Id: I4650478274a87adfed91fe53097c1e47fb2f10c7
Signed-off-by: Fernando Guzman Lugo <fernando.guzman.lugo@intel.com>
Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Irina Patru <irina.patru@intel.com>
Signed-off-by: Yong Yao <yong.yao@intel.com>