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/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index 9217e1c..e9626a2 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -381,9 +381,10 @@
 	partitionProps.Vbmeta_partitions = vbmetaPartitions
 
 	deviceProps := &filesystem.DeviceProperties{
-		Main_device:       proptools.BoolPtr(true),
-		Ab_ota_updater:    proptools.BoolPtr(ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.AbOtaUpdater),
-		Ab_ota_partitions: ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.AbOtaPartitions,
+		Main_device:               proptools.BoolPtr(true),
+		Ab_ota_updater:            proptools.BoolPtr(ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.AbOtaUpdater),
+		Ab_ota_partitions:         ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.AbOtaPartitions,
+		Ab_ota_postinstall_config: ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.AbOtaPostInstallConfig,
 	}
 	if bootloader, ok := f.createBootloaderFilegroup(ctx); ok {
 		deviceProps.Bootloader = proptools.StringPtr(":" + bootloader)