fs_mgr_overlayfs: Try "/system" and "/" when setting "/system" shared type

b/239574953 uncovers an obscure test device configuration that breaks
with aosp/2146960.

Without aosp/2146960, "/system" is not a mountpoint, because after
switch_root, the "/system" mount entry becomes "/", thus changing the
subtree propagation type of "/system" would fail.
With aosp/2146960, "/system" is bind-mounted to itself after
switch_root, ensuring "/system" being a mountpoint, thus changing the
subtree propagation type of "/system" is allowed.

Before we can re-land aosp/2146960, just try both "/system" and "/"
when changing the subtree shared propagation type of "/system", so
both scenarios are handled.

Test: Add submount under /system and adb remount
Change-Id: I4006a5c1b1987d5f6452efa069ec5c7d2ac7c8ec