Check if fresco is initialised before clearing memory caches

Summary:
Fixes the following crash:
```
Fatal Exception: java.lang.RuntimeException: Unable to destroy activity {com.example/com.example.MainActivity}: java.lang.NullPointerException: ImagePipelineFactory was not initialized!
       at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3831)
       at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3849)
       at android.app.ActivityThread.access$1500(ActivityThread.java:150)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1398)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:148)
       at android.app.ActivityThread.main(ActivityThread.java:5417)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:764)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
Caused by java.lang.NullPointerException: ImagePipelineFactory was not initialized!
       at com.facebook.common.internal.Preconditions.checkNotNull(Preconditions.java:226)
       at com.facebook.imagepipeline.core.ImagePipelineFactory.getInstance(ImagePipelineFactory.java:74)
       at com.facebook.drawee.backends.pipeline.Fresco.getImagePipelineFactory(Fresco.java:92)
       at com.facebook.drawee.backends.pipeline.Fresco.getImagePipeline(Fresco.java:97)
       at com.facebook.react.modules.fresco.FrescoModule.onHostDestroy(FrescoModule.java:186)
       at com.facebook.react.bridge.ReactContext.onHostDestroy(ReactContext.java:240)
       at com.facebook.react.ReactInstanceManager.moveToBeforeCreateLifecycleState(ReactInstanceManager.java:667)
       at com.facebook.react.ReactInstanceManager.onHostDestroy(ReactInstanceManager.java:586)
       at com.facebook.react.ReactInstanceManager.onHostDestroy(ReactInstanceManager.java:599)
       at com.facebook.react.ReactActivityDelegate.onDestroy(ReactActivityDelegate.java:142)
       at com.facebook.react.ReactActivity.onDestroy(ReactActivity.java:72)
       at android.app.Activity.performDestroy(Activity.java:6456)
       at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1143)
       at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3818)
       at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3849)
       at android.app.ActivityThread.access$1500(ActivityThread.java:150)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1398)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:148)
       at android.app.ActivityThread.main(ActivityThread.java:5417)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:764)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
```

This was introduced by d9ae27ba89

1. Create app with an image to be loaded
2. Background app before fresco has been initialised. (Very tight window to do this)
3. Should not crash

cc foghina
Closes https://github.com/facebook/react-native/pull/14359

Differential Revision: D5508505

Pulled By: hramos

fbshipit-source-id: 5a66d594625783f1c30180fe78c5baddb4f835aa

ReactAndroid/src/main/java/com/facebook/react/modules/fresco/FrescoModule.java

@@ -182,7 +182,7 @@ public class FrescoModule extends ReactContextBaseJavaModule implements
     // According to the javadoc for LifecycleEventListener#onHostDestroy, this is only called when
     // the 'last' ReactActivity is being destroyed, which effectively means the app is being
     // backgrounded.
-    if (mClearOnDestroy) {
+    if (hasBeenInitialized() && mClearOnDestroy) {
       Fresco.getImagePipeline().clearMemoryCaches();
     }
   }