style: Only apply neutral entry focus color on lock screen

Commit a72c95de changed the focus style for all system entries to a more neutral color, as the stronger focus often clashes with the wallpaper background on the lock screen.

However that is not a concern for entries that appear over the neutral system background. Worse, the neutral focus indication provides so little contrast with the background there that is not much of an indication anymore.

Address this by restoring the old focus indication for the generic system_entry mix-in, but override it specifically on the lock screen to avoid the clash with the wallpaper background.

Fixes a72c95de Closes #6945 (closed) Closes #6880 (closed)