Add conversations image resource

- Conversations image resource is a compilation of the conversation
icons that are allowed to bypass DND
- Update category header text on DND > Display options for hidden notifications
page
- Update a11y bug in messages and calls pref pages that said images were
tappable

Test: manual
Bug: 151845457
Change-Id: I5387722abf0543b6e07378d498cc0085122c91b9

res/layout/zen_mode_senders_overlay_image.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2020 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.
+-->
+<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+                android:layout_width="match_parent"
+                android:layout_height="wrap_content">
+    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+                    android:id="@+id/zen_mode_settings_senders_overlay_view"
+                    android:layout_centerInParent="true"
+                    android:layout_width="wrap_content"
+                    android:layout_height="wrap_content">
+    </RelativeLayout>
+</RelativeLayout>