Provide developer APIs to discover application-specific paths on
secondary external storage devices. Covers files, cache, and OBB
directories. Apps will not have write access outside their package-
specific directories on secondary devices, so only primary storage is
exposed through Environment.
Creation of .nomedia files will be handled by FUSE daemon in future
change.
Change-Id: Ifcce6201a686d80269d7285adb597c008cf8fa7c