Make letterbox education dialog react to dark/light themes

Update letterbox education dialog to use colours assigned based on
the system theme.
Force letterbox education dialog component to be recreated if the system theme
has been change or device is docked while the dialoge is visible

Fixes: 270936761
Test: atest WMShellUnitTests:LetterboxEduWindowManagerTest
Change-Id: I3422c4ca7d7abdc085b28b341f7d539bc1447f66