axolotl: Move PRODUCT_SHIPPING_API_LEVEL to device.mk

Otherwise it's not being picked up by the build system.

Change-Id: Ib7a29661b520a64571c61739d443e74c794e9d8c
diff --git a/device.mk b/device.mk
index 7b5aa6d..b8dbdfa 100644
--- a/device.mk
+++ b/device.mk
@@ -462,6 +462,9 @@
     android.hardware.sensors@2.0.vendor \
     libsensorndkbridge \
 
+# Shipping API
+PRODUCT_SHIPPING_API_LEVEL := 29
+
 # Telephony
 PRODUCT_PACKAGES += \
     ims-ext-common \