Future-proof the frontlight handling on Kindle.

Guard against trying to get any kind of frontlight info on devices
without one.

So far the flIntensity prop has always been there, even on devices
without one, but better not assume that'll always be the case ;).

That, and be consistent with the other lipc_handle tests.

frontend/device/kindle/powerd.lua

@@ -15,10 +15,12 @@ function KindlePowerD:init()
     if lipc then
         self.lipc_handle = lipc.init("com.github.koreader.kindlepowerd")
     end
-    if self.lipc_handle then
+    if self.device.hasFrontlight() then
-        self.flIntensity = self.lipc_handle:get_int_property("com.lab126.powerd", "flIntensity")
+        if self.lipc_handle ~= nil then
-    else
+            self.flIntensity = self.lipc_handle:get_int_property("com.lab126.powerd", "flIntensity")
-        self.flIntensity = self:read_int_file(self.fl_intensity_file)
+        else
+            self.flIntensity = self:read_int_file(self.fl_intensity_file)
+        end
     end
 end