Make testrunner runnable from art directory

We were using a relative path to the soong_ui.bash script in
testrunner. This meant testrunner could only be run if you were in the
ANDROID_BUILD_TOP directory. This makes that path absolute by
prepending ANDROID_BUILD_TOP to it.

Test: ./test/testrunner/testrunner.py --host -b
Change-Id: I9007d2de79b07bf77a6ddc6a569f553acb20f63e
diff --git a/test/testrunner/testrunner.py b/test/testrunner/testrunner.py
index d1ba93f..4e873c1 100755
--- a/test/testrunner/testrunner.py
+++ b/test/testrunner/testrunner.py
@@ -1012,7 +1012,7 @@
       build_targets += 'test-art-target-run-test-dependencies '
     if 'jvm' in _user_input_variants['target']:
       build_targets += 'test-art-host-run-test-dependencies '
-    build_command = 'build/soong/soong_ui.bash --make-mode'
+    build_command = env.ANDROID_BUILD_TOP + '/build/soong/soong_ui.bash --make-mode'
     build_command += ' DX='
     build_command += ' ' + build_targets
     if subprocess.call(build_command.split()):