New "sm" shell tool to call StorageManager.

Surface basic StorageManager commands through shell tool, like
simple listing of disks and volumes, and commands like mounting and
partitioning.

The output is designed to be parsed by host-side testing tools,
instead of relying on fragile dumpsys parsing.

Bug: 19993667
Change-Id: I993e92ecf57996678965945f0ae648b392a77ea2