tuna: simplify /factory partition handling

Currently, we do the following sequence on every bootup:
1. mount /factory ro (fstab)
2. remount /factory as rw
3. update selinux labels on the partition
4. remount /factory as ro

By mounting /factory in rw mode from the fstab, we can avoid the second step,
simplifying the bootup sequence.

Also replaced the restorecons with one restorecon_recursive,
which does the same task in one line.

And lastly, removed some unnecessary chown/chmods - remounting
does not overwrite the permissions and the owner.

Conflicts:
	rootdir/fstab.tuna
tuna: remove unnecessary init entries

From Android 5.0, these are present in the main init.rc file.
tuna: drop restorecons from init.tuna.rc file

Recent changes have obsoleted the need to call
restorecon on any of /data files and dirs. This
patch drops those restorecons from the file.