Reboot sequence: Unmount active apexes before unmounting /data

Having mounted apexes with loop back devices backing files on /data
partition will prevent clean unmount of it. Unmounting them and tearing
down loop devices should minimize the risk of that.

Note that it won't fix the issue completely, as there are a few (~2-3)
processes that keep restarting even after SIGKILL is sent. Which means
that they can still hold references to apexes on /data partition. But
in practice probability of this is quite low.

Test: adb reboot
Test: put tzdata apex in /data/apex/active && adb reboot
Bug: 158152940
Change-Id: I4624567b3d0f304dba4c6e37b77abd89e57411de