[fix, Android] Don't steal frontlight control on start (#3319)

I believe this should be `if isKobo()`, or better yet that the entire
block should be moved to `KoboPowerD:init()` because afaik that is the
only platform where the system doesn't provide trustworthy frontlight
information. But to be absolutely sure that I don't break anything (and I
don't want to spend any time on this atm) I'm temporarily excluding only
Android where this behavior is known to be problematic.

See discussion in https://github.com/koreader/koreader/issues/3118#issuecomment-334995879

References #3118 (using keyword "references" because phrases like "possibly fixes"
result in GH autoclose).

frontend/device/generic/powerd.lua

@@ -21,10 +21,20 @@ function BasePowerD:new(o)
     if o.device and o.device.hasFrontlight() then
         o.fl_intensity = o:frontlightIntensityHW()
         o:_decideFrontlightState()
-        if o:isFrontlightOn() then
-            o:turnOnFrontlightHW()
-        else
-            o:turnOffFrontlightHW()
+        -- Note added by @Frenzie 2017-10-08
+        -- I believe this should be `if isKobo()`, or better yet that the entire
+        -- block should be moved to `KoboPowerD:init()` because afaik that is the
+        -- only platform where the system doesn't provide trustworthy frontlight
+        -- information. But to be absolutely sure that I don't break anything (and I
+        -- don't want to spend any time on this atm) I'm temporarily excluding only
+        -- Android where this behavior is known to be problematic.
+        -- See discussion in https://github.com/koreader/koreader/issues/3118#issuecomment-334995879
+        if not o.device:isAndroid() then
+            if o:isFrontlightOn() then
+                o:turnOnFrontlightHW()
+            else
+                o:turnOffFrontlightHW()
+            end
         end
     end
     return o

spec/unit/frontlight_spec.lua

@@ -12,7 +12,10 @@ describe("Frontlight function in PowerD", function()
             fl_min = 1,
             fl_max = 5,
             device = {
-                hasFrontlight = function() return true end
+                hasFrontlight = function() return true end,
+                -- TODO @Frenzie remove this once possibly turning on frontlight
+                -- on init is Kobo-only; see device/generic/powerd 2017-10-08
+                isAndroid = function() return false end,
             },
         }
     end)