Fix AndroidSchedulers to create an instance using hook only once.

Previously we were asking the hook for an instance every time `mainThread()` was called. This not only impacted performance, but also broke the contract the that hook was a factory–instead requiring that it behave like a thread-safe, lazily-initialized instance cache.

If for whatever reason you do need to change the instance over time, return a scheduler instance which delegates to another and allows swapping out the delegate.

rxandroid/src/main/java/rx/android/schedulers/AndroidSchedulers.java

@@ -16,24 +16,28 @@
 import android.os.Looper;
 import rx.Scheduler;
 import rx.android.plugins.RxAndroidPlugins;
+import rx.android.plugins.RxAndroidSchedulersHook;
 
 /** Android-specific Schedulers. */
 public final class AndroidSchedulers {
+    private static final AndroidSchedulers INSTANCE = new AndroidSchedulers();
+
+    private final Scheduler mainThreadScheduler;
+
     private AndroidSchedulers() {
-        throw new AssertionError("No instances");
-    }
+        RxAndroidSchedulersHook hook = RxAndroidPlugins.getInstance().getSchedulersHook();
 
-    // See https://github.com/ReactiveX/RxAndroid/issues/238
-    // https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom
-    private static class MainThreadSchedulerHolder {
-        static final Scheduler MAIN_THREAD_SCHEDULER = new LooperScheduler(Looper.getMainLooper());
+        Scheduler main = hook.getMainThreadScheduler();
+        if (main != null) {
+            mainThreadScheduler = main;
+        } else {
+            mainThreadScheduler = new LooperScheduler(Looper.getMainLooper());
+        }
     }
 
     /** A {@link Scheduler} which executes actions on the Android UI thread. */
     public static Scheduler mainThread() {
-        Scheduler scheduler =
-                RxAndroidPlugins.getInstance().getSchedulersHook().getMainThreadScheduler();
-        return scheduler != null ? scheduler : MainThreadSchedulerHolder.MAIN_THREAD_SCHEDULER;
+        return INSTANCE.mainThreadScheduler;
     }
 
     /** A {@link Scheduler} which executes actions on {@code looper}. */