Add sticker support.

No sticker packs are available for use yet, but we now have the
latent ability to send and receive.

res/drawable/transfer_controls_background.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+
+    <corners android:radius="16dp" />
+    <solid android:color="@color/core_grey_02" />
+
+</shape>