ART: Use ANDROID_HOST_OUT for ANDROID_ROOT in Runtime tests

On the host, if ANDROID_ROOT is not set, first try ANDROID_HOST_OUT.
This will ensure that builds with OUT_DIR_COMMON_BASE will run the
gtests correctly, as icu will use ANDROID_ROOT to look for a file.

Change-Id: I8988901be32249c1c79f96b36947a647df5de041