Change the remount command to be more container-friendly

Passing in MS_REMOUNT | MS_BIND tells the kernel to only remove the
MS_RDONLY flag and keep all the other flags the same. This is also the
only kind of remount that is allowed in Linux containers (to be more
precise, within user namespaces).

This change also attempts to always run the remount command when dealing
with /, since containers will almost always run a loop device, and since
the number of the device changes, it is not convenient to put it in the
fstab. Plus, the container won't have permission to modify it, but might
be able to perform the remount.

Bug: 72178046
Test: `adb remount` works in both sailfish and Chrome OS
Change-Id: I9e8ec8afcd57f67875a312824667768b3aa89faa