Pocketbook: Keep wifi alive as long as wifi is enabled (#9208)

Schedule wifi keep-alive on wifi start but do not reschedule when
connection is lost. This will make sure this code is not run when
wifi is disabled again saving some battery and cpu cycles.

frontend/device/pocketbook/device.lua

@@ -334,10 +334,28 @@ function PocketBook:reboot()
 end
 
 function PocketBook:initNetworkManager(NetworkMgr)
+    local UIManager = require("ui/uimanager")
+
+    local function keepWifiAlive()
+        -- Make sure only one wifiKeepAlive is scheduled
+        UIManager:unschedule(keepWifiAlive)
+
+        if NetworkMgr:isWifiOn() then
+            logger.dbg("ping wifi keep alive and reschedule")
+
+            inkview.NetMgrPing()
+            UIManager:scheduleIn(30, keepWifiAlive)
+        else
+            logger.dbg("wifi is disabled do not reschedule")
+        end
+    end
+
     function NetworkMgr:turnOnWifi(complete_callback)
         inkview.WiFiPower(1)
-        if inkview.NetConnect(nil) ~= C.NET_OK then
-            logger.info('NetConnect failed')
+        if inkview.NetConnect(nil) == C.NET_OK then
+            keepWifiAlive()
+        else
+            logger.info("NetConnect failed")
         end
         if complete_callback then
             complete_callback()