build: add OMNI_PRODUCT_PROPERTIES hook

allows injecting properties into FINAL_PRODUCT_PROPERTIES overriding
previous values easily

Change-Id: If9645df2dcf80c0ce665c960962df73f5458e479
(cherry picked from commit 8efe84e87adc41b405b99afa702da20cc0370dc6)
diff --git a/core/sysprop.mk b/core/sysprop.mk
index e4cdc72..4563acb 100644
--- a/core/sysprop.mk
+++ b/core/sysprop.mk
@@ -325,6 +325,7 @@
 # Order matters here. When there are duplicates, the last one wins.
 # TODO(b/117892318): don't allow duplicates so that the ordering doesn't matter
 _prop_vars_ := \
+    OMNI_PRODUCT_PROPERTIES \
     ADDITIONAL_PRODUCT_PROPERTIES \
     PRODUCT_PRODUCT_PROPERTIES