Support `enabled` flag in product variable config

Some Android.bp modules have `enabled: false` but only use a product
variable such as `source_build` to enable the module.  Currently b2build
does not handle this case at all. This commit adds the functionality
to support this use case.

Also, remove `__enabled` suffix in ProductVariable SelectKey.

Bug: 210546943
Test: go test ./bp2build
Topic: use_enabled_flag_product_variable_config
Change-Id: I459c17a84c172df010666391066bf4d11d19253e
diff --git a/android/module.go b/android/module.go
index c2fa848..5053720 100644
--- a/android/module.go
+++ b/android/module.go
@@ -1139,20 +1139,71 @@
 		}
 	}
 
-	data.Append(required)
+	productConfigEnabledLabels := []bazel.Label{}
+	if !proptools.BoolDefault(enabledProperty.Value, true) {
+		// If the module is not enabled by default, then we can check if a
+		// product variable enables it
+		productConfigEnabledLabels = productVariableConfigEnableLabels(ctx)
 
-	var err error
-	constraints := constraintAttributes{}
-	constraints.Target_compatible_with, err = enabledProperty.ToLabelListAttribute(
+		if len(productConfigEnabledLabels) > 0 {
+			// In this case, an existing product variable configuration overrides any
+			// module-level `enable: false` definition
+			newValue := true
+			enabledProperty.Value = &newValue
+		}
+	}
+
+	productConfigEnabledAttribute := bazel.MakeLabelListAttribute(bazel.LabelList{
+		productConfigEnabledLabels, nil,
+	})
+
+	platformEnabledAttribute, err := enabledProperty.ToLabelListAttribute(
 		bazel.LabelList{[]bazel.Label{bazel.Label{Label: "@platforms//:incompatible"}}, nil},
 		bazel.LabelList{[]bazel.Label{}, nil})
-
 	if err != nil {
-		ctx.ModuleErrorf("Error processing enabled attribute: %s", err)
+		ctx.ModuleErrorf("Error processing platform enabled attribute: %s", err)
 	}
+
+	data.Append(required)
+
+	constraints := constraintAttributes{}
+	moduleEnableConstraints := bazel.LabelListAttribute{}
+	moduleEnableConstraints.Append(platformEnabledAttribute)
+	moduleEnableConstraints.Append(productConfigEnabledAttribute)
+	constraints.Target_compatible_with = moduleEnableConstraints
+
 	return constraints
 }
 
+// Check product variables for `enabled: true` flag override.
+// Returns a list of the constraint_value targets who enable this override.
+func productVariableConfigEnableLabels(ctx *topDownMutatorContext) []bazel.Label {
+	productVariableProps := ProductVariableProperties(ctx)
+	productConfigEnablingTargets := []bazel.Label{}
+	const propName = "Enabled"
+	if productConfigProps, exists := productVariableProps[propName]; exists {
+		for productConfigProp, prop := range productConfigProps {
+			flag, ok := prop.(*bool)
+			if !ok {
+				ctx.ModuleErrorf("Could not convert product variable %s property", proptools.PropertyNameForField(propName))
+			}
+
+			if *flag {
+				axis := productConfigProp.ConfigurationAxis()
+				targetLabel := axis.SelectKey(productConfigProp.SelectKey())
+				productConfigEnablingTargets = append(productConfigEnablingTargets, bazel.Label{
+					Label: targetLabel,
+				})
+			} else {
+				// TODO(b/210546943): handle negative case where `enabled: false`
+				ctx.ModuleErrorf("`enabled: false` is not currently supported for configuration variables. See b/210546943", proptools.PropertyNameForField(propName))
+			}
+		}
+	}
+
+	return productConfigEnablingTargets
+}
+
 // A ModuleBase object contains the properties that are common to all Android
 // modules.  It should be included as an anonymous field in every module
 // struct definition.  InitAndroidModule should then be called from the module's