vold: support more than 1 UMS share

Used for devices with both mountable internal and external sdcards.

This is a forward-port and clean up of the 10.2 code (which contains
a lot of deadcode.)

Original authors:

    Alex Zepeda <alex@inferiorhumanorgans.com>
    Genokolar <genokolar@gmail.com>
    Josh Stone <cuviper@gmail.com>
    mik_os <kiril.mik.os@gmail.com>
    Oran Avraham <oranav@gmail.com>
    Pawit Pornkitprasan <p.pawit@gmail.com>
    Tony Layher <layhertony@gmail.com>

Change-Id: I655d1ea5f48ad7f83b2bb9e6e0385fbeb13f3bd2