Implement android ServerValue.TIMESTAMP

Author: BlooJeans

android/src/main/java/io/fullstack/firestack/FirestackModule.java

@@ -178,9 +178,13 @@ public String setKeyOrDefault(
 
   @ReactMethod
   public void serverValue(@Nullable final Callback onComplete) {
+    WritableMap timestampMap = Arguments.createMap();
+    for (Map.Entry<String, String> entry : ServerValue.TIMESTAMP.entrySet()) {
+      timestampMap.putString(entry.getKey(), entry.getValue());
+    }
+
     WritableMap map = Arguments.createMap();
-    // TODO
-    map.putString("TIMESTAMP", "ServerValue.TIMESTAMP");
+    map.putMap("TIMESTAMP", timestampMap);
     onComplete.invoke(null, map);
   }