Refactor compare_images

1. Rename some variables to more detail
2. Fix a bug that occurs when system.img contains product, system_ext
3. Remove multithread code, it made performance worse :(
  before: 136.97s user 1070.22s system 290% cpu 6:55.84 total
  after:  56.29s user 76.95s system 105% cpu 2:06.84 total

Bug: 138329983
Test: compare some images, and check if result is same as before
Change-Id: I4cec27475283a34d063ad3251cfc5909c34071c1
1 file changed