Add flag for themed App Headers in desktop windowing

To guard changes to the App Header styling, particularly around
background and foreground color changes based on combinations of:
- System dark/light theme
- App dark/light theme
- Focused state
- Default vs Custom headers active

Bug: 328668781
Test: m
Change-Id: I3c4789dba11493bbab72beb0b59e225983ebc299