Add test_mainline_modules to the auto-gen test config(GTest only).

To support parameterized mainline modules in Test Mapping, we plan to
add a new parameter called test_mainline_modules in build system to
auto-generate the test config based on the parameter.

For detailed information: go/test-mapping-mainline-gcl
(serach for auto-generated pattern)

Bug: 155238134
Test: add "test_mainline_modules: [some.apk]" to libstatspull_test,
and build the modules, confirm the parameterized option is added
in the test config.

Change-Id: I31d6dfbb71881d7a7026cf2f36ba6ca6a97870ad

tradefed/autogen.go

@@ -64,12 +64,16 @@ type Config interface {
 
 type Option struct {
 	Name  string
+	Key   string
 	Value string
 }
 
 var _ Config = Option{}
 
 func (o Option) Config() string {
+	if o.Key != "" {
+		return fmt.Sprintf(`<option name="%s" key="%s" value="%s" />`, o.Name, o.Key, o.Value)
+	}
 	return fmt.Sprintf(`<option name="%s" value="%s" />`, o.Name, o.Value)
 }