Keep strong reference to OnSharedPreferenceChangeListener (#131)

Summary:
A strong reference to the listener should be kept when calling `registerOnSharedPreferenceChangeListener`, otherwise the listener can get garbage-collected and stop working.

Mentioned in the Android SDK docs [here](https://developer.android.com/reference/android/content/SharedPreferences.html#registerOnSharedPreferenceChangeListener(android.content.SharedPreferences.OnSharedPreferenceChangeListener)):

> Caution: The preference manager does not currently store a strong reference to the listener. You must store a strong reference to the listener, or it will be susceptible to garbage collection. We recommend you keep a reference to the listener in the instance data of an object that will exist as long as you need the listener.
Closes https://github.com/facebook/Sonar/pull/131

Differential Revision: D8751484

Pulled By: xiphirx

fbshipit-source-id: e46715d637c89491770c266fc22c27db41beb042

android/plugins/sharedpreferences/SharedPreferencesSonarPlugin.java

@@ -23,6 +23,24 @@ public class SharedPreferencesSonarPlugin implements SonarPlugin {
 
   private SonarConnection mConnection;
   private final SharedPreferences mSharedPreferences;
+  private final SharedPreferences.OnSharedPreferenceChangeListener
+      onSharedPreferenceChangeListener =
+          new SharedPreferences.OnSharedPreferenceChangeListener() {
+            @Override
+            public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
+              if (mConnection == null) {
+                return;
+              }
+              mConnection.send(
+                  "sharedPreferencesChange",
+                  new SonarObject.Builder()
+                      .put("name", key)
+                      .put("deleted", !mSharedPreferences.contains(key))
+                      .put("time", System.currentTimeMillis())
+                      .put("value", mSharedPreferences.getAll().get(key))
+                      .build());
+            }
+          };
 
   /**
    * Creates a {@link android.content.SharedPreferences} plugin for Sonar
@@ -54,23 +72,7 @@ public class SharedPreferencesSonarPlugin implements SonarPlugin {
    */
   public SharedPreferencesSonarPlugin(Context context, String name, int mode) {
     mSharedPreferences = context.getSharedPreferences(name, mode);
-
+    mSharedPreferences.registerOnSharedPreferenceChangeListener(onSharedPreferenceChangeListener);
-    mSharedPreferences.registerOnSharedPreferenceChangeListener(
-        new SharedPreferences.OnSharedPreferenceChangeListener() {
-          @Override
-          public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
-            if (mConnection != null) {
-              mConnection.send(
-                  "sharedPreferencesChange",
-                  new SonarObject.Builder()
-                      .put("name", key)
-                      .put("deleted", !mSharedPreferences.contains(key))
-                      .put("time", System.currentTimeMillis())
-                      .put("value", mSharedPreferences.getAll().get(key))
-                      .build());
-            }
-          }
-        });
   }
 
   @Override