Add date time, checks on android_root, etc.

* Dump current date/time to the output .html file
  to distinguish files from different build dates.
* Guess android_root from the path of warn_common.py,
  but only if it contains build/make and build/soong.
* Do not depend on the TOP variable,
  which is no longer emitted by build/soong make.
* Do not depend on the USE_RBE variables,
  which is not emitted in local makes.

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: Ib75c60216f9e06f9a768e63b536495fb9658843f