Add vendor to list of supported partitions in gen_build_prop.py

The kati built vendor.img will still use the kati built vendor
build.prop file, but the soong built vendor.img will use an
autogenerated `build_prop` module for vendor

This CL should be a noop for now

Bug: 374371755
Test: diff in contents for aosp_cf_x86_64_phone https://paste.googleplex.com/5766413037076480
Summary
- timestamp change (expected)
- ro.hwui.use_vulkan changes from "" to false, but I think this is
  functionally equivalent
Test: presubmits

Change-Id: I10bc5b6e8d48b86fa70d5ab0722fee658b1a224a
diff --git a/android/build_prop.go b/android/build_prop.go
index ede93ed..120ad94 100644
--- a/android/build_prop.go
+++ b/android/build_prop.go
@@ -65,6 +65,9 @@
 		return ctx.Config().ProductPropFiles(ctx)
 	} else if partition == "odm" {
 		return ctx.Config().OdmPropFiles(ctx)
+	} else if partition == "vendor" {
+		// TODO (b/375500423): Add android-info.txt to prop files
+		return ctx.Config().VendorPropFiles(ctx)
 	}
 	return nil
 }
@@ -104,6 +107,7 @@
 	"system_ext",
 	"product",
 	"odm",
+	"vendor",
 }
 
 func (p *buildPropModule) GenerateAndroidBuildActions(ctx ModuleContext) {
diff --git a/android/config.go b/android/config.go
index 1e51840..feed22f 100644
--- a/android/config.go
+++ b/android/config.go
@@ -2195,6 +2195,10 @@
 	return PathsForSource(ctx, c.productVariables.OdmPropFiles)
 }
 
+func (c *config) VendorPropFiles(ctx PathContext) Paths {
+	return PathsForSource(ctx, c.productVariables.VendorPropFiles)
+}
+
 func (c *config) ExtraAllowedDepsTxt() string {
 	return String(c.productVariables.ExtraAllowedDepsTxt)
 }
diff --git a/android/variable.go b/android/variable.go
index c352942..6693d91 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -521,6 +521,7 @@
 	SystemExtPropFiles []string `json:",omitempty"`
 	ProductPropFiles   []string `json:",omitempty"`
 	OdmPropFiles       []string `json:",omitempty"`
+	VendorPropFiles    []string `json:",omitempty"`
 
 	EnableUffdGc *string `json:",omitempty"`