[fs-management] Pass block device by handle, not fd

When launching filesystem binaries, the following handles
are now passable:

FS_HANDLE_ROOT_ID: The handle representing the root of the filesystem
server.
FS_HANDLE_BLOCK_DEVICE_ID: The handle representing the block device
of the target filesystem.

Moving forward, it is expected that both mounters and mountees (the
filesystems themselves) will begin using the block device more "by
handle", rather than "by fd". This avoids interfacing with ulib/fdio,
simplifies transmission of FIDL requests, and encourages asynchronous
communication.

Tested by "fs-management" and "fs-tests", which invoke
the mount, mkfs, and fsck commands, achieving coverage
of the code paths touched by this patch.

ZX-2718 #done

Change-Id: Ia6ad9d69c8a13e92189125f590d7e25c125ea027