SystemUI: Follow light/dark theme in power menu

Now that we've modified the power menu to refresh on UI mode changes,
make it follow the system light/dark theme for better integration in
light mode.

SystemUI: Always refresh power menu on UI mode change

This is necessary for reliably theming the global actions dialog with
dynamic colors and adapting it to light/dark themes.

Demo screenshots (with color overlays applied):
https://twitter.com/kdrag0n/status/1445960685427433473



[kdrag0n: ported to 12L]

Change-Id: If58fb4079a4cd11414ff928fad3576beecb14ff5
Signed-off-by: althafvly <althafvly@gmail.com>