Show icons for allowed sounds

(Some icons are temporary, until we get the final assets)

Also some fixes/improvements to CircularIconsPreference:
* Show the correct placeholder and +N icons.
* Fix the displayIcons-before-measure case (global layout listener was incorrect).
* Properly cancel pending image load futures (field didn't point to the actual future).
* Don't reload icons if it's the same set (depends on equals() for the items, so unfortunately doesn't work for AppEntry yet).

Test: atest com.android.settings.notification.modes
Bug: 346551087
Flag: android.app.modes_ui
Change-Id: I9d029a5fdd785ada4e2ba4d8a90eba72b5fb9085

res/drawable/preference_circular_icons_plus_item_background.xml

@@ -0,0 +1,28 @@
+<!--
+  ~ Copyright (C) 2024 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.
+  -->
+
+<shape
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:androidprv="http://schemas.android.com/apk/prv/res/android"
+    android:shape="oval">
+    <size
+        android:width="@dimen/zen_mode_circular_icon_diameter"
+        android:height="@dimen/zen_mode_circular_icon_diameter" />
+    <solid android:color="?androidprv:attr/materialColorSecondaryContainer" />
+    <!-- TODO: b/346551087 - Include border (or not) according to final design
+    <stroke android:width="1dp" android:color="?androidprv:attr/materialColorOnSecondaryContainer" />
+    -->
+</shape>