DeviceListener:onToggleFrontlight: add notification of "unchanged"

[hasezoey]

add notification when the frontlight is unchanged (no callback is called)

because in 2023.04 a notification would be always be shown (like Frontlight disabled) and since 2023.05 no notification would be shown anymore if the frontlight did not change (no intensity change), see

koreader/frontend/device/generic/powerd.lua

Line 149 in d350418

if self.fl_intensity == self.fl_min then return false end --- @fixme what the hell?

re #10722

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This change is 

[pazos]

Pinging @NiLuJe, who worked recently on it (c.f #10597)

[NiLuJe]

I stand by that quoted comment, but looks harmless, yeah ;).

[NiLuJe]

Description is slightly misleading, though, it doesn't really have anything to do with the callback being called or not, the only cases where it returns false are explicitly that insane min test quoted, or if the fl is already on or off (depending).

[NiLuJe]

or if the fl is already on or off (depending).

And that doesn't really apply to the "toggle" use-cases, but for direct "on"/"off" calls... which I'm not sure is even a thing that can be done by a user ;p.

[NiLuJe]

Have I mentioned how much I hate the FL backend? ;p.