Fix duplicate suspend scheduling in Device:onPowerEvent

We should always unschedule suspend before scheduling it again (i.e.,
use rescheduleSuspend ;)).

Fix #8097 (many thanks to @Mel-kior for the detailed repro!)

frontend/device/generic/device.lua

@@ -341,7 +341,7 @@ function Device:onPowerEvent(ev)
             -- Only actually schedule suspension if we're still supposed to go to sleep,
             -- because the Wi-Fi stuff above may have blocked for a significant amount of time...
             if self.screen_saver_mode then
-                UIManager:scheduleIn(self.suspend_wait_timeout, self.suspend)
+                self:rescheduleSuspend()
             end
         end)
     end

frontend/ui/uimanager.lua

@@ -1178,16 +1178,15 @@ function UIManager:_checkTasks()
     -- task.action may schedule other events
     self._task_queue_dirty = false
     while true do
-        local nu_task = #self._task_queue
-        if nu_task == 0 then
-            -- all tasks checked
+        if #self._task_queue == 0 then
+            -- Nothing to do!
             break
         end
-        local task = self._task_queue[1]
-        local task_tv = task.time or TimeVal.zero
+        local next_task = self._task_queue[1]
+        local task_tv = next_task.time or TimeVal.zero
         if task_tv <= self._now then
             -- remove from table
-            table.remove(self._task_queue, 1)
+            local task = table.remove(self._task_queue, 1)
             -- task is pending to be executed right now. do it.
             -- NOTE: be careful that task.action() might modify
             -- _task_queue here. So need to avoid race condition
@@ -1195,7 +1194,7 @@ function UIManager:_checkTasks()
         else
             -- queue is sorted in ascendant order, safe to assume all items
             -- are future tasks for now
-            wait_until = task.time
+            wait_until = next_task.time
             break
         end
     end