Add @keep to HiddenLifecycleReference so that the flutter_android_lifecycle package can access via reflection instead of compile-time references.

Author: matthew-carroll

shell/platform/android/io/flutter/embedding/engine/plugins/lifecycle/HiddenLifecycleReference.java

@@ -5,6 +5,7 @@
 package io.flutter.embedding.engine.plugins.lifecycle;
 
 import android.arch.lifecycle.Lifecycle;
+import android.support.annotation.Keep;
 import android.support.annotation.NonNull;
 
 /**
@@ -16,6 +17,7 @@
  * {@link Lifecycle} in a way that makes it easier for Flutter and the Flutter plugin ecosystem to
  * handle breaking changes in Lifecycle libraries.
  */
+@Keep
 public class HiddenLifecycleReference {
   @NonNull
   private final Lifecycle lifecycle;