Adds support for 'ignored-on-host'

Adds a filter mechanism that can exclude property values from being
included in the common value extraction. That is needed to prevent the
snapshot mechanism from generating invalid output for properties that
are ignored on host (and have their values cleared) and which are not
tagged with `android:"arch_variant"`.

Changes:
* Updates the documentation of SdkMemberType to explain what effect
  the 'ignored-on-host' tag has.
* Adds some tests for this new mechanism.

Bug: 155628860
Test: m nothing
Change-Id: I7ebd333079619dba546bc8c4911d567e0287b676

sdk/update.go

@@ -1215,11 +1215,26 @@ func (s *sdk) getPossibleOsTypes() []android.OsType {
 // struct (or one of its embedded structs).
 type fieldAccessorFunc func(structValue reflect.Value) reflect.Value
 
+// Checks the metadata to determine whether the property should be ignored for the
+// purposes of common value extraction or not.
+type extractorMetadataPredicate func(metadata propertiesContainer) bool
+
+// Indicates whether optimizable properties are provided by a host variant or
+// not.
+type isHostVariant interface {
+	isHostVariant() bool
+}
+
 // A property that can be optimized by the commonValueExtractor.
 type extractorProperty struct {
 	// The name of the field for this property.
 	name string
 
+	// Filter that can use metadata associated with the properties being optimized
+	// to determine whether the field should be ignored during common value
+	// optimization.
+	filter extractorMetadataPredicate
+
 	// Retrieves the value on which common value optimization will be performed.
 	getter fieldAccessorFunc
 
@@ -1273,6 +1288,20 @@ func (e *commonValueExtractor) gatherFields(structType reflect.Type, containingS
 			continue
 		}
 
+		var filter extractorMetadataPredicate
+
+		// Add a filter
+		if proptools.HasTag(field, "sdk", "ignored-on-host") {
+			filter = func(metadata propertiesContainer) bool {
+				if m, ok := metadata.(isHostVariant); ok {
+					if m.isHostVariant() {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
 		// Save a copy of the field index for use in the function.
 		fieldIndex := f
 
@@ -1304,6 +1333,7 @@ func (e *commonValueExtractor) gatherFields(structType reflect.Type, containingS
 		} else {
 			property := extractorProperty{
 				name,
+				filter,
 				fieldGetter,
 				reflect.Zero(field.Type),
 				proptools.HasTag(field, "android", "arch_variant"),
@@ -1372,6 +1402,12 @@ func (e *commonValueExtractor) extractCommonProperties(commonProperties interfac
 
 	for _, property := range e.properties {
 		fieldGetter := property.getter
+		filter := property.filter
+		if filter == nil {
+			filter = func(metadata propertiesContainer) bool {
+				return true
+			}
+		}
 
 		// Check to see if all the structures have the same value for the field. The commonValue
 		// is nil on entry to the loop and if it is nil on exit then there is no common value or
@@ -1389,6 +1425,15 @@ func (e *commonValueExtractor) extractCommonProperties(commonProperties interfac
 			itemValue := reflect.ValueOf(container.optimizableProperties())
 			fieldValue := fieldGetter(itemValue)
 
+			if !filter(container) {
+				expectedValue := property.emptyValue.Interface()
+				actualValue := fieldValue.Interface()
+				if !reflect.DeepEqual(expectedValue, actualValue) {
+					return fmt.Errorf("field %q is supposed to be ignored for %q but is set to %#v instead of %#v", property, container, actualValue, expectedValue)
+				}
+				continue
+			}
+
 			if commonValue == nil {
 				// Use the first value as the commonProperties value.
 				commonValue = &fieldValue