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.

+ DownloadProvider: Check if the device actually supports second storage.

Change-Id: I7995ae63ec5769103d7ad66bcd677e383ef7f440