Device:Android: call "_decideFrontlightState" to keep "is_fl_on" in sync

[hasezoey]

re #10725

This PR is so that variable is_fl_on is also kept in-sync when fl_intensity is being set, this could maybe be de-duplicated by always calling it after the both if's.

Why?

because thing that depend on is_fl_on do not work correctly after just fl_intensity being set, like BasePowerD:isFrontlightOn (by extension also BasePowerD:isFrontlightOff) and so in turn things like BasePowerD:toggleFrontlight call the wrong function.

Example:
(assuming #10726 is merged, makes it easier to explain)

initial state:
fl_intensity: 50
is_fl_on: true

then you go to the lights dialog, change brightness to 0 (fronlight being off) and then just fl_intensity is set to 0, but is_fl_on stays true - which is wrong, and so if you then do something like event ToggleFrontlight it will go the path of Frontlight Disabled, even though the frontlight was already disabled

@pazos i hope this examples makes it more clear what i meant with #10725 (comment), i have go the path of or to use powerd:_decideFrontlightState()
to make it less forgettable in the future maybe refactor this with one of the other options listed in the mentioned comment (like adding a helper function for only internal state)

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This change is 

[hasezoey]

just to be on the same page, because of (#10725 (comment))

I'm not against fixing issues, but I would like to keep workarounds buried in android-luajit-launcher. If the entire problem here is is_fl_on just make a method as part on the LightsController class and let drivers deal with it.

koreader/frontend/device/generic/powerd.lua

Line 20 in d350418

is_fl_on = false, -- whether the frontlight is on

koreader/frontend/device/generic/powerd.lua

Lines 103 to 105 in d350418

	function BasePowerD:isFrontlightOn()
	return self.is_fl_on
	end

koreader/frontend/device/generic/powerd.lua

Lines 125 to 132 in d350418

	function BasePowerD:toggleFrontlight(done_callback)
	if not self.device:hasFrontlight() then return false end
	if self:isFrontlightOn() then
	return self:turnOffFrontlight(done_callback)
	else
	return self:turnOnFrontlight(done_callback)
	end
	end

[pazos]

Ok, thanks for the lengthy explanation.

Now makes sense to me.

[NiLuJe]

The first hunk makes sense (c.f., e.g., BasePowerD's init, or even simply setIntensity), but I'd stash the second one away in the imp.

[NiLuJe]

I would possibly move that one inside Android's setIntensityHW implementation. (We do the same on a few other platforms, e.g., Kobo).