Frameworks: Fix reboot when formatting SDCard

MountService is not aware of /storage/emulated/legacy path which is
causing the issue on ExternalStorageFormatter service. We are getting
the a list of non-emulated SD cards and format the first one .

Change-Id: Iba1a9f6e40499f7f6fb095a5ee4e711b02bed439
CRs-fixed: 489037