APIs for multiple external storage devices.

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