Allow remounting /mnt/user/0 as slave mount

This is required for Idf851b3a42910e0ce8fdd75daea1cce91dd1aa98
And is part of enabling upcoming platform changes that are
described in the bug linked below.

Bug: 135341433
Test: m
Change-Id: I1d842fcfae3740d51e7cb2050decf1f83543af7e
diff --git a/public/init.te b/public/init.te
index f7ef232..4d7da55 100644
--- a/public/init.te
+++ b/public/init.te
@@ -80,7 +80,7 @@
 
 # Create and mount on directories in /.
 allow init rootfs:dir create_dir_perms;
-allow init { rootfs cache_file cgroup storage_file system_data_file system_file vendor_file postinstall_mnt_dir }:dir mounton;
+allow init { rootfs cache_file cgroup storage_file mnt_user_file system_data_file system_file vendor_file postinstall_mnt_dir }:dir mounton;
 allow init cgroup_bpf:dir { create mounton };
 
 # Mount bpf fs on sys/fs/bpf