Set CHECKOUT_SOURCE_ROOT environment variable for Android test wrapper.

This makes it possible to clean up the recipes a lot.

BUG=
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34409004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7837 4adac7df-926f-26a2-2b94-8c16560cd09d