Allow special access to priv-app directories

Allow DownloadsProvider and ExternalStorageProvider to access private
app directories in secondary volumes.

Bug: 159688123
Test: manual
Change-Id: Icf330b783625fed02e585914c171547d5441f646
Merged-In: Icf330b783625fed02e585914c171547d5441f646
(cherry picked from commit 3f6130004ef32b5c66afebf68a01daea47441f3c)
2 files changed