[android]: fix backlight toggle (#6520)

* check on startup if frontlight can be changed.
* Init fl_min if necessary not to use intensity==0

frontend/device/android/powerd.lua

@@ -6,17 +6,37 @@ local AndroidPowerD = BasePowerD:new{
     fl_max = 100,
 }
 
+-- Let the footer know of the change
+local function broadcastLightChanges()
+    if package.loaded["ui/uimanager"] ~= nil then
+        local Event = require("ui/event")
+        local UIManager = require("ui/uimanager")
+        UIManager:broadcastEvent(Event:new("FrontlightStateChanged"))
+    end
+end
+
 function AndroidPowerD:frontlightIntensityHW()
     return math.floor(android.getScreenBrightness() / self.bright_diff * self.fl_max)
 end
 
 function AndroidPowerD:setIntensityHW(intensity)
+    -- if frontlight switch was toggled of, turn it on
+    android:enableFrontlightSwitch()
+
     self.fl_intensity = intensity
     android.setScreenBrightness(math.floor(intensity * self.bright_diff / self.fl_max))
 end
 
 function AndroidPowerD:init()
     self.bright_diff = android:getScreenMaxBrightness() - android:getScreenMinBrightness()
+
+    -- if necessary scale fl_min:
+    --    do not use fl_min==0 if getScreenMinBrightness!=0,
+    --    because intenstiy==0 would mean to use system intensity
+    if android:getScreenMinBrightness() ~= self.fl_min then
+        self.fl_min = math.ceil(android:getScreenMinBrightness() * self.bright_diff / self.fl_max)
+    end
+
     if self.device:hasNaturalLight() then
         self.warm_diff = android:getScreenMaxWarmth() - android:getScreenMinWarmth()
         self.fl_warmth = self:getWarmth()
@@ -42,4 +62,23 @@ function AndroidPowerD:isChargingHW()
     return android.isCharging()
 end
 
+function AndroidPowerD:turnOffFrontlightHW()
+    if not self:isFrontlightOnHW() then
+        return
+    end
+    android.setScreenBrightness(self.fl_min)
+    self.is_fl_on = false
+    broadcastLightChanges()
+end
+
+function AndroidPowerD:turnOnFrontlightHW()
+    if self:isFrontlightOn() and self:isFrontlightOnHW() then
+        return
+    end
+    android.setScreenBrightness(math.floor(self.fl_intensity * self.bright_diff / self.fl_max))
+
+    self.is_fl_on = true
+    broadcastLightChanges()
+end
+
 return AndroidPowerD

frontend/device/kobo/powerd.lua

@@ -348,6 +348,7 @@ function KoboPowerD:turnOffFrontlightHW()
         UIManager:broadcastEvent(Event:new("FrontlightStateChanged"))
     end
 end
+
 function KoboPowerD:turnOnFrontlightHW()
     -- NOTE: Insane workaround for the first toggle after a startup with the FL off.
     -- The light is actually off, but hw_intensity couldn't have been set to a sane value because of a number of interactions.