DownloadProvider: add to support download into sdcard storage

When insert into downloads database to start a new download,
will check file uri destination is valid or not, currently it
only support phone storage downloading, what we do is to add
supporting sdcard storage downloading, this will make download
save path setting feature work well.

Change-Id: I7995ae63ec5769103d7ad66bcd677e383ef7f440
DownloadProvider: Check if the device actually supports second storage.

Change-Id: I78297ca235e058aecfd0ff5a0e990fc3004b4c77