Handle more mixed warning lines from RBE

* Use android_root or __file__ path to find
  the source tree root and its sub directories.
* Use the sub directory list to clean up RBE-returned
  warning lines, removing all mixed prefix before
  the top sub directories.
* Change some re.match rules and order to save
  unnecessary comparisons.

Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Test: warn.py --gencsv build.log > warnings.csv
Bug: 198657613
Change-Id: I78cc17d04b8ab9e12935ef04797f3272298d5267