Split warn.py into multiple files

* Split warning patterns into *_warn_patterns.py
* Split project list into android_project_list.py
* Split out the Severity class, to be changed later
* Split core of warn.py into warn_common.py and
  leave only platform dependent code in warn.py.
  This allows the core logic be used with different
  parallel processing libraries.
* Old warn.py just calls -m warn.warn.

Test: path_to_build/tools/warn.py build.log
Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Change-Id: I6734e4472a21018cd5ce06d549f6dbca24f4de54