Private Tabs theme on Android

In #24920 (moved), we use Private Tabs by default on Android. Unfortunately, Fennec couple the browsing mode and theme, where normal tabs use the light theme and private tabs use the dark theme. This is different from Desktop where the dark theme is only used when it is enabled, otherwise the light theme is used for both normal and PBM.

How should we resolve this difference between desktop and Android? See [ticket:24920#comment:9 Georg's comment] (1) about this for a little more context.