Factor out common code in Java warn patterns.

* Some comments are kept to reduce diff artifacts.
* Warning patterns will be separated by tools later,
  to make it easier to share code with Chromium and
  other build tools.

Bug: 141493530
Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Change-Id: I6b589478cf5ae73940b4712d9faabd0909546f02