Avoid secondary background changing in split view

We previously used `tertiarySystemBackground` as our dark mode colour for `posSecondaryBackground`.

This split view support work has shown that in dark mode, Apple’s tertiary background actually changes colour when in split view.

Since we don’t use system colours for our other backgrounds, this can result in views becoming indistinguishable from one another.

This commit defines the dark mode secondary background to match the system background in non-splitview mode.

Author: joshheald

WooCommerce/Classes/POS/Utils/Color+WooCommercePOS.swift

@@ -32,7 +32,7 @@ extension Color {
         Color(
             UIColor(
                 light: .white,
-                dark: .tertiarySystemBackground
+                dark: UIColor(red: 44.0 / 255.0, green: 44.0 / 255.0, blue: 46.0 / 255.0, alpha: 1.0)
             )
         )
     }