Have the `art` script honor a pre-set `ANDROID_RUNTIME_ROOT`.

This is so that users/environments (e.g. Golem) may override the
default value set by the `art` script.

Test: Rely on Golem
Bug: 121117762
Bug: 119293618
Bug: 113373927
Change-Id: Id8c7500828d735a229a396cf77e84584862fc9b8
1 file changed