Make ext4 userdata checkpoints work with metadata encryption

When both ext4 user data checkpoints and metadata encryption are
enabled, we are creating two stacked dm devices. This had not been
properly thought through or debugged.

Test: Enable metadata encryption on taimen (add
keydirectory=/metadata/vold/metadata_encryption to flags for userdata in
fstab.hardware)
    Unfortunately metadata is not wiped by fastboot -w, so it is
    necessary to rm metadata/vold -rf whenever you wipe data.
    fastboot flashall -w works
    fastboot reboot -w works
    A normal boot works
    Disable checkpoint commits with
    setprop persist.vold.dont_commit_checkpoint 1
    vdc checkpoint startCheckpoint 10
    adb reboot
    wait for device to fully boot then
    adb reboot
    Wait for device to fully boot then
    adb logcat -d | grep Checkpoint shows the rollback in the logs

    This tests encryption on top of checkpoints with commit, encryption
    without checkpoints, and rollback, which seems to be the key cases.

Bug: 135905679
Change-Id: I8365a40298b752af4bb10d00d9ff58ce04beab1f