Iteration on multi-user external storage.

Define /storage as top-level concept, so that we enforce permissions
uniformly.  Moves external storage paths from headers to per-device
environment variables.  Added missing mount flags, and we no longer
have adb-specific external storage.

Bug: 6925012
Change-Id: Ic7ca953be2f552d3f0ec9e69f89fef751daa1b29