A filesystem manager library to mount filesystems for init.

Instead of specifying in init what to mount, and having various hacks in init
itself to deal with encryption, use a filesystem manager library to do the
work, that can also be invoked by vold when mounting an encrypted volume.
Keep all the magic filesystem info an a device specific fstab file.

Change-Id: Ib988f1e4fb0638ba1d5fd98407fa6d8cf862aaca