Cleanup bp2build-progress.py

Not a thorough refactor, just a surface-level
cleanup I did while attempting to understand
the script.

Test: changed .join(dot_entries) to .join(sorted(dot_entries)), then:
      ./scripts/bp2build-progress/bp2build-progress.py graph -m com.android.media.swcodec --use-queryview > after.dot
      git checkout m/master
      ./scripts/bp2build-progress/bp2build-progress.py graph -m com.android.media.swcodec --use_queryview=true > before.dot
      diff before.dot after.dot
      And got no differences
Change-Id: Ic612c0be608a11e7bdb49f1135bb8abbbf971ac7