[fix, Android] hasEinkScreen should be a function (#4557)

Fixes https://github.com/koreader/koreader/pull/4541#issuecomment-461739983

frontend/device/android/device.lua

@@ -13,7 +13,7 @@ local Device = Generic:new{
     hasKeys = yes,
     hasDPad = no,
     isAndroid = yes,
-    hasEinkScreen = android.isEink(),
+    hasEinkScreen = function() return android.isEink() end,
     hasFrontlight = yes,
     firmware_rev = android.app.activity.sdkVersion,
     display_dpi = android.lib.AConfiguration_getDensity(android.app.config),