fs_mgr_overlayfs: Fix "/" mount propagation type after remount

If remounting "/system" caused "/" to be set to MS_PRIVATE, restore it
to MS_SHARED after remount.

Bug: 306124139
Test: adb-remount-test
Test: remount /system and verify /proc/self/mountinfo
Change-Id: I12bc2ccdaf7d2b2de330064b61a8533b96defe00