Add android::fs_mgr namespace for new Fstab code

Also add libfstab dependencies where needed.  Previously the
`typedef struct FstabEntry Volume;` line served to both define a
`struct FstabEntry` as well as alias Volume to it.  With the new
namespace for android::fs_mgr::FstabEntry, `struct FstabEntry` isn't
compatible anymore, so we need to alias Volume to the real
android::fs_mgr::FstabEntry.

In doing so, we need to include <fstab/fstab.h> and this requires
libfstab as a library, which a few modules did not have before.

Test: treehugger
Change-Id: I655209a0efb304b3e0568db0748bd5cf7cecbdb7