Auto generate userdata.img

Test: CI and unit tests
Bug: 377563861
Change-Id: I42582db31377600412d2acf0dceab5ee9e15bc58
diff --git a/fsgen/fsgen_mutators.go b/fsgen/fsgen_mutators.go
index 92ea128..e530a5b 100644
--- a/fsgen/fsgen_mutators.go
+++ b/fsgen/fsgen_mutators.go
@@ -93,6 +93,9 @@
 	if ctx.DeviceConfig().BuildingOdmImage() && ctx.DeviceConfig().OdmPath() == "odm" {
 		generatedPartitions = append(generatedPartitions, "odm")
 	}
+	if ctx.DeviceConfig().BuildingUserdataImage() && ctx.DeviceConfig().UserdataPath() == "data" {
+		generatedPartitions = append(generatedPartitions, "userdata")
+	}
 	if ctx.Config().ProductVariables().PartitionVarsForSoongMigrationOnlyDoNotUse.BuildingSystemDlkmImage {
 		generatedPartitions = append(generatedPartitions, "system_dlkm")
 	}
@@ -144,6 +147,7 @@
 					"com.android.vndk.v34": defaultDepCandidateProps(ctx.Config()),
 				},
 				"system_dlkm": {},
+				"userdata":    {},
 			},
 			fsDepsMutex:               sync.Mutex{},
 			moduleToInstallationProps: map[string]installationProperties{},