testrunner/env.py - Run from any directory

env.py will calculate ANDROID_BUILD_TOP relative to where its located,
if it wasn't already set.

This enables 'run_build_test_target' to successfully run even if
build/envsetup.sh was not run first.

(also rename env.py's members to have the private naming convention,
as they are not meant to be exposed to other modules)

Change-Id: I236c6005d56fa1a155c6eb104936e3e010a169de