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/prebuilt_build_tool.go b/android/prebuilt_build_tool.go
index 516d042..e5edf91 100644
--- a/android/prebuilt_build_tool.go
+++ b/android/prebuilt_build_tool.go
@@ -86,7 +86,7 @@
 
 func (t *prebuiltBuildTool) MakeVars(ctx MakeVarsModuleContext) {
 	if makeVar := String(t.properties.Export_to_make_var); makeVar != "" {
-		if t.Target().Os != BuildOs {
+		if t.Target().Os != ctx.Config().BuildOS {
 			return
 		}
 		ctx.StrictRaw(makeVar, t.toolPath.String())