Set the Soong config variable to keep building ART from source.

Without this Soong may run into inconsistencies due to stale prebuilts.
This amends https://r.android.com/1529658.

Test: TH build of NDK
Bug: 175918603
Bug: 172480615
Change-Id: I14680e4547d7d5f560d31ef61b71af8d159c4c53