extract_utils: support extracting directly from an ota zip

  * Specify the ota zip name as the only parameter to extract-files.sh
  * Will extract to $CM_ROOT/system_dump
  * Bail out on A/B OTA zips. We cannot support these.
  * Handles block based OTA zips by using sdat2img.py
  * Store the zip's MD5 and check if its already extracted. If so, don't
    bother extracting again

Change-Id: I03038e38dac51e6cb60d493c7e6362754d1daf02