Disable jack usage for when building RS prebuilts

Bug: 22693954
Change-Id: Ibfc46acf2a23e711b9f4c8236bff4d7c9f8acb43
diff --git a/update_rs_prebuilts.sh b/update_rs_prebuilts.sh
index 3a5115e..397d919 100755
--- a/update_rs_prebuilts.sh
+++ b/update_rs_prebuilts.sh
@@ -39,6 +39,10 @@
 # actual required compiler pieces.
 export FORCE_BUILD_RS_COMPAT=true
 
+# Disable JACK when buiding RS prebuilts. Without this variable, we won't be
+# able to get classes.jar.
+export UPDATE_RS_PREBUILTS_DISABLE_JACK=true
+
 # ANDROID_HOST_OUT is where the new prebuilts will be constructed/copied from.
 ANDROID_HOST_OUT=$MY_ANDROID_DIR/out/host/$SHORT_OSNAME-x86/