[Android, UX] Add Device:openLink() implementation (#4824)

Fixes https://github.com/koreader/koreader/issues/4821

Same as https://github.com/koreader/koreader/pull/4822

Thanks to @pazos who implemented the Android backend, see https://github.com/koreader/android-luajit-launcher/pull/133

frontend/device/android/device.lua

@@ -27,6 +27,10 @@ local Device = Generic:new{
     display_dpi = android.lib.AConfiguration_getDensity(android.app.config),
     hasClipboard = yes,
     hasOTAUpdates = canUpdateApk,
+    openLink = function(self, link)
+        if not link or type(link) ~= "string" then return end
+        return android.openLink(link) == 0
+    end,
     --[[
     Disable jit on some modules on android to make koreader on Android more stable.