Replace android.BuildOs with Config.BuildOS

Replace the android.BuildOs constant with Config.BuildOS so that it
can vary based on the product config.

Bug: 190084016
Test: all Soong tests
Change-Id: Ia67f872d8b2ab788747a22e3a9659dc21c9775cd
diff --git a/android/test_suites.go b/android/test_suites.go
index 6b7b909..22f6cf2 100644
--- a/android/test_suites.go
+++ b/android/test_suites.go
@@ -60,7 +60,7 @@
 	for _, module := range SortedStringKeys(files) {
 		installedPaths = append(installedPaths, files[module]...)
 	}
-	testCasesDir := pathForInstall(ctx, BuildOs, X86, "testcases", false).ToMakePath()
+	testCasesDir := pathForInstall(ctx, ctx.Config().BuildOS, X86, "testcases", false).ToMakePath()
 
 	outputFile := PathForOutput(ctx, "packaging", "robolectric-tests.zip")
 	rule := NewRuleBuilder(pctx, ctx)