Add PRODUCT_EXTRA_OTA_KEYS to add extra ota keys in otacerts.zip

Equivalent to PRODUCT_EXTRA_RECOVERY_KEYS but for A/B OTA.

Bug: 211848136
Test: set PRODUCT_EXTRA_OTA_KEYS and check otacerts.zip
Change-Id: I81e27d12a22b405f6227b09c01ed684dfcede19e
diff --git a/core/product.mk b/core/product.mk
index 04c59ff..b069dda 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -183,6 +183,7 @@
 # signing tools can substitute them for the test key embedded by
 # default.
 _product_list_vars += PRODUCT_OTA_PUBLIC_KEYS
+_product_list_vars += PRODUCT_EXTRA_OTA_KEYS
 _product_list_vars += PRODUCT_EXTRA_RECOVERY_KEYS
 
 # Should we use the default resources or add any product specific overlays