fs_mgr: Split libfs_mgr and libfstab

The goal is to make the header definitions of the two curiously
intertwined libraries less chaotic.

After this change, libfstab's header would be self contained. In the
sense that all symbols exported by its headers are defined in its
compilation units.
libfs_mgr would still embed libfstab like before, it can use internal
symbols (symbols not exported by public headers) of libfstab through
the libfstab/fstab_priv.h private header.

Keep include_fstab/ as a symbolic link pointing to its new location.
This is a temporary workaround as there are still some bad build rules
(incorrectly) depending on the old include path with Android.bp
`include_dirs` directive.

Bug: 293695109
Test: build
Change-Id: Ib70a84984ac2cbfca5f5b27fadebf6a16e58146a