init.rc: Remount / with MS_REMOUNT|MS_BIND

Since we only want to change the ro flag on / (and leave all other mount
flags alone), this can also be achieved by passing MS_REMOUNT|MS_BIND,
even if the mount is not a bind-mount.

This aims to make running Android within a user namespace easier, since
remounts without the MS_BIND flag are forbidden.

Bug: 73255020
Test: aosp_sailfish still boots
Test: rootfs on / type rootfs (rw,seclabel)
      /dev/root on / type ext4 (ro,seclabel,relatime,data=ordered)
Change-Id: I2f89a8badfc467db47304c9355648e8fd8ad1272