Annotate webvtt package with @NonNullApi

PiperOrigin-RevId: 277916734

library/core/src/main/java/com/google/android/exoplayer2/text/webvtt/package-info.java

+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * 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.
+ *
+ */
+@NonNullApi
+package com.google.android.exoplayer2.text.webvtt;
+
+import com.google.android.exoplayer2.util.NonNullApi;