fs_mgr: remount: pull root device from fstab

For devices that do not support overlayfs that are system as root,
need to pull /dev/root device from / mount point in fstab.

Test: adb-remount-test.sh
Bug: 126256072
Change-Id: I198c0e8c60770508737318afde95db2966294ff5
1 file changed