Prevent unspecified values in soong_config_string_variable

Currently, setting a soong config string variable to a value
that's not listed just does nothing. Change it so it's an
error instead.

Bug: 220375749
Test: Presubmits
Change-Id: I14c1c76ac556ccb0cbd167c95eb3fd7625a6a52c

android/soongconfig/modules.go

@@ -639,9 +639,13 @@ func (s *stringVariable) initializeProperties(v reflect.Value, typ reflect.Type)
 // Extracts an interface from values containing the properties to apply based on config.
 // If config does not match a value with a non-nil property set, the default value will be returned.
 func (s *stringVariable) PropertiesToApply(config SoongConfig, values reflect.Value) (interface{}, error) {
+	configValue := config.String(s.variable)
+	if configValue != "" && !InList(configValue, s.values) {
+		return nil, fmt.Errorf("Soong config property %q must be one of %v, found %q", s.variable, s.values, configValue)
+	}
 	for j, v := range s.values {
 		f := values.Field(j)
-		if config.String(s.variable) == v && !f.Elem().IsNil() {
+		if configValue == v && !f.Elem().IsNil() {
 			return f.Interface(), nil
 		}
 	}
@@ -858,3 +862,13 @@ type emptyInterfaceStruct struct {
 }
 
 var emptyInterfaceType = reflect.TypeOf(emptyInterfaceStruct{}).Field(0).Type
+
+// InList checks if the string belongs to the list
+func InList(s string, list []string) bool {
+	for _, s2 := range list {
+		if s2 == s {
+			return true
+		}
+	}
+	return false
+}