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