Add Ab_ota_postinstall_config prop to android_device

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

Test: Built soong target_files.zip locally
Bug: 388633394
Change-Id: I6f9f574fc3720b611b4c2df36b303857ccb9f14d
diff --git a/android/variable.go b/android/variable.go
index aace6ed..4e1df3c 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -666,6 +666,7 @@
 	AbOtaUpdater                      bool                                     `json:",omitempty"`
 	AbOtaPartitions                   []string                                 `json:",omitempty"`
 	AbOtaKeys                         []string                                 `json:",omitempty"`
+	AbOtaPostInstallConfig            []string                                 `json:",omitempty"`
 
 	// Avb (android verified boot) stuff
 	BoardAvbEnable          bool                                `json:",omitempty"`