Signal-Android/app/src/main/res/layout/mediarail_button_item.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:tools="http://schemas.android.com/tools"
    tools:viewBindingIgnore="true"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="56dp"
    android:layout_height="56dp"
    android:layout_margin="2dp"
    android:background="@drawable/mediarail_button_background">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:src="@drawable/ic_plus_28"
        android:elevation="4dp" />

</FrameLayout>