Add memory pressure listener in ReactHost

Summary:
Add memory pressure listener for ReactHost and ReactInstance

Changelog: [Internal]

Reviewed By: ejanzer

Differential Revision: D23214241

fbshipit-source-id: ec8476aeda8d9781d918ea41e5cab69fa862996e

ReactAndroid/src/main/java/com/facebook/react/MemoryPressureRouter.java

@@ -20,7 +20,7 @@ public class MemoryPressureRouter implements ComponentCallbacks2 {
   private final Set<MemoryPressureListener> mListeners =
       Collections.synchronizedSet(new LinkedHashSet<MemoryPressureListener>());
 
-  MemoryPressureRouter(Context context) {
+  public MemoryPressureRouter(Context context) {
     context.getApplicationContext().registerComponentCallbacks(this);
   }