Make variant headers as arch variant

Make cc_api_variant's header property as arch variant, so headers can be
differed by arch variant

Bug: 244244438
Test: Panther vendorimage build succeeded
Change-Id: I400301793209c9e0214fc118cd4981194d507fbd

cc/library_stub.go

@@ -288,8 +288,8 @@ type ccApiexportProperties struct {
 }
 
 type variantExporterProperties struct {
-	// Header directory or library to export
-	Export_headers []string
+	// Header directory to export
+	Export_headers []string `android:"arch_variant"`
 
 	// Export all headers as system include
 	Export_headers_as_system *bool