Add ab_ota_keys prop to android_device

This will be populated with PRODUCT_OTA_PUBLIC_KEYS for now.

It will be used to generate otakeys.txt file in
target_files.zip

Test: Built soong target_files.zip locally
Bug: 388633394
Change-Id: If10e0f6a7fcd55239e87dffa92341ed8c31d17cc
diff --git a/android/variable.go b/android/variable.go
index 419f778..aace6ed 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -665,6 +665,7 @@
 	ProductVirtualAbCowVersion        string                                   `json:",omitempty"`
 	AbOtaUpdater                      bool                                     `json:",omitempty"`
 	AbOtaPartitions                   []string                                 `json:",omitempty"`
+	AbOtaKeys                         []string                                 `json:",omitempty"`
 
 	// Avb (android verified boot) stuff
 	BoardAvbEnable          bool                                `json:",omitempty"`