Run "plugins" singleton in parallel with the rest.

This completes the changes in aosp/q/topic:"parallel-singletons".

(cherry picked from commit 30bf7fe32d0ba644cd181645fab3c714a9a11b2f)

Bug: 281536768
Test: manual, presubmits
Change-Id: I6529e957fbcca6c18441f823a00061db7556e303
diff --git a/android/plugin.go b/android/plugin.go
index c9d1338..4672453 100644
--- a/android/plugin.go
+++ b/android/plugin.go
@@ -29,7 +29,7 @@
 }
 
 func RegisterPluginSingletonBuildComponents(ctx RegistrationContext) {
-	ctx.RegisterSingletonType("plugins", pluginSingletonFactory)
+	ctx.RegisterParallelSingletonType("plugins", pluginSingletonFactory)
 }
 
 // pluginSingleton is a singleton to handle allowlisting of the final Android-<product_name>.mk file