Create Umbrella Header for FIAMDisplay (#1857)

* Create Umbrella Header for FIAMDisplay

This is causing an issue while trying to build the zip file.

* Append .h to filename.

Author: ryanwilson

Firebase/InAppMessagingDisplay/Public/FirebaseInAppMessagingDisplay.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import "FIRIAMDefaultDisplayImpl.h"