Don't emit make vars from disabled modules

Previously, disabled modules were able to emit make vars and this could
cause duplicated make var definitions when a prebuilt_build_tools is
enabled for both linux_glibc and linux_bionic.

Bug: 159685774
Test: m
Change-Id: I8570ae5aabb8199e7c3901921886bcfa849037fe

android/makevars.go

@@ -234,7 +234,7 @@ func (s *makeVarsSingleton) GenerateBuildActions(ctx SingletonContext) {
 	}
 
 	ctx.VisitAllModules(func(m Module) {
-		if provider, ok := m.(ModuleMakeVarsProvider); ok {
+		if provider, ok := m.(ModuleMakeVarsProvider); ok && m.Enabled() {
 			mctx := &makeVarsContext{
 				SingletonContext: ctx,
 			}