first_stage_mount: Bind mount /system after switch root

This ensures /system to be a mountpoint, which allows adb-remount to
change the subtree propagation type of /system and MS_MOVE submounts
under /system.

Test: Add submount under /system and adb remount
Change-Id: Ib281be41eaf86bc3e6b7dfac893e5e27b8374da8