[SES-2555] Add bg to image loading

app/src/main/res/drawable/image_loading_background.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+
+    <solid android:color="?backgroundSecondary" />
+
+    <corners android:radius="?dialogCornerRadius" />
+
+</shape>

app/src/main/res/layout/thumbnail_view.xml

@@ -5,7 +5,7 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:background="@color/transparent_black_6">
+    android:background="@drawable/image_loading_background">
 
     <ImageView
         android:id="@+id/thumbnail_image"