Add test_mainline_module option.

Bug: 165425972

Test: m HelloWorldTests
Change-Id: I6e60b3895328c4f7aef0bfb30dcdefc30ee1d8ed
diff --git a/cc/androidmk.go b/cc/androidmk.go
index c899cdd..380b4e9 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -377,6 +377,7 @@
 		if !BoolDefault(test.Properties.Auto_gen_config, true) {
 			entries.SetBool("LOCAL_DISABLE_AUTO_GENERATE_TEST_CONFIG", true)
 		}
+		entries.AddStrings("LOCAL_TEST_MAINLINE_MODULES", test.Properties.Test_mainline_modules...)
 	})
 
 	androidMkWriteTestData(test.data, ctx, entries)