Handle warning lines from RBE

* Recognize USE_RBE special environment variable.
  * Remove RBE special file path prefix /b/f/w/
    and any leading characters.
  * Only do this for android now, maybe for chrome later.
* When finding android_root, do not use
  /b/f/w/ and /tmp/ file paths.
* Recognize BUILD_ID and add it into .html output.
* Style improvement with continue statements.

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: I57746f026d5d8b71e792127b8d6ba15f6b28103c