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

android/variable.go

@@ -601,10 +601,16 @@ func (p *ProductConfigProperty) SelectKey() string {
 
 	value := p.FullConfig
 	if value == p.Name {
-		value = "enabled"
+		value = ""
 	}
-	// e.g. acme__feature1__enabled, android__board__soc_a
-	return strings.ToLower(strings.Join([]string{p.Namespace, p.Name, value}, "__"))
+
+	// e.g. acme__feature1, android__board__soc_a
+	selectKey := strings.ToLower(strings.Join([]string{p.Namespace, p.Name}, "__"))
+	if value != "" {
+		selectKey = strings.ToLower(strings.Join([]string{selectKey, value}, "__"))
+	}
+
+	return selectKey
 }
 
 // ProductConfigProperties is a map of maps to group property values according