Get Android TOP from build.log file.

find_android_root works only when warn.py is found
so it works for build bot, but not when warn.py is
run against a stand alone build.log file.
Now get the TOP directory path from build.log
if it is found.

Bug: 37725427
Test: warn.py --gencsv build.log
Change-Id: I7b776c2e71a8d8d769f0600b08abe37acbd6ce7c