Rename the headless system user flag to under 32 chars.

Test: build
Bug: 131601785
Change-Id: I0ca7218a26a0feff7861399bf25cc895e434b940
diff --git a/car_product/build/car.mk b/car_product/build/car.mk
index d72ed42..628bb01 100644
--- a/car_product/build/car.mk
+++ b/car_product/build/car.mk
@@ -53,7 +53,7 @@
 PRODUCT_PROPERTY_OVERRIDES += \
     persist.bluetooth.enablenewavrcp=false \
     ro.carrier=unknown \
-    ro.fw.multiuser.headless_system_user=true
+    ro.fw.mu.headless_system_user=true
 
 # Overlay for Google network and fused location providers
 $(call inherit-product, device/sample/products/location_overlay.mk)