Allow Scoped Directory Access on whole volume.

There are some scenarios where an app needs access to the whole SD Card,
not subdirectories. For example, user might have a SDCard with
directories like vacation_pictures (instead of Pictures/vacation);
another example is a file management app.

BUG: 27676858
Change-Id: I20ef713de7e4dfa7e2d7d07bab11898af186d673