Fix the MapWithLoopDevice

This fix is to keep the dm line in the status file and let the
UnmapImageDevice to clean up correctly.

Bug: 171861574
Test: execute following command on a device with a SD card inserted
    adb shell am start-activity \
        -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
        -a android.os.image.action.START_INSTALL \
        -d file:///storage/emulated/0/Download/system.raw.gz \
        --el KEY_SYSTEM_SIZE $(du -b system.raw|cut -f1) \
        --el KEY_USERDATA_SIZE 4294967296

Change-Id: Ia56f8f724f04e7e20586e088c89b62a1068766e4