Make Android getTriggerValueForKey consistent with iOS

- `getTriggerValueForKey` returns a rejected promise in the iOS bridge if there is NO value for the key specified

- This commit makes the Android bridge behave the same way so users don't receive an error returned in iOS while receiving a returned value of `null` in Android

Author: nan-li

android/src/main/java/com/geektime/rnonesignalandroid/RNOneSignal.java

@@ -594,7 +594,13 @@ public void removeTriggersForKeys(ReadableArray keys) {
 
    @ReactMethod
    public void getTriggerValueForKey(String key, Promise promise) {
-      promise.resolve(OneSignal.getTriggerValueForKey(key));
+      Object val = OneSignal.getTriggerValueForKey(key);
+      if (val == null) {
+         Log.e("OneSignal", "getTriggerValueForKey: There was no value for the key: " + key);
+         promise.reject("No Value", "There was no value for the key: " + key);
+         return;
+      }
+      promise.resolve(val);
    }
 
    /* in app message click */