Fix incorrect override of PRODUCT_PRODUCT_PROPERTIES

Change-Id: I2adaf8b63e6965ffe76f3765b981072f05ac2eb4
diff --git a/shared/device.mk b/shared/device.mk
index 8c6748a..1e52385 100644
--- a/shared/device.mk
+++ b/shared/device.mk
@@ -35,7 +35,7 @@
 # Properties that are not vendor-specific. These will go in the product
 # partition, instead of the vendor partition, and do not need vendor
 # sepolicy
-PRODUCT_PRODUCT_PROPERTIES := \
+PRODUCT_PRODUCT_PROPERTIES += \
     persist.adb.tcp.port=5555 \
     persist.traced.enable=1 \
     ro.com.google.locationfeatures=1 \