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

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/selected_indicator"
    android:layout_width="56dp"
    android:layout_height="56dp"
    android:alpha="0"
    app:layout_constraintBottom_toBottomOf="@id/image_container"
    app:layout_constraintEnd_toEndOf="@id/image_container"
    app:layout_constraintStart_toStartOf="@id/image_container"
    app:layout_constraintTop_toTopOf="@id/image_container"
    tools:alpha="1"
    tools:showIn="@layout/media_overview_detail_item_media">

    <ImageView
        android:layout_width="21dp"
        android:layout_height="21dp"
        android:layout_gravity="bottom|end"
        android:background="@drawable/circle_tintable"
        android:contentDescription="@string/MediaOverviewActivity_Selected_description"
        android:padding="1dp"
        android:scaleType="centerInside"
        app:backgroundTint="@color/signal_background_primary"
        app:srcCompat="@drawable/ic_check_circle_solid_20" />

</FrameLayout>