Allows prebuilts in override_apex.

Bug: 197787336
Test: Create an override_apex that uses a different `prebuilts` than its
      base. Observe built contents.
Test: apex_test.go
Change-Id: I7666ed6cfe3f2fa5dd81e5f8c1961477dabbbd3c

android/override_module.go

@@ -295,7 +295,7 @@ func performOverrideMutator(ctx BottomUpMutatorContext) {
 }
 
 func overridableModuleDepsMutator(ctx BottomUpMutatorContext) {
-	if b, ok := ctx.Module().(OverridableModule); ok {
+	if b, ok := ctx.Module().(OverridableModule); ok && b.Enabled() {
 		b.OverridablePropertiesDepsMutator(ctx)
 	}
 }