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
DownloadProvider: fix secondary storage support

Previous attempt at "add to support download into sdcard storage"
was a steaming pile of feces. This is re-implementation
of that commit with better aroma and behavior.

Change-Id: If9aba4c73b24d052b0379d624302ffda03fdf8e4
Merge pull request #2 from AOSPA-legacy/kitkat

pulling changes from aospa-legacy