AutoSuspend: Don't send LeaveStandby events from a zombie plugin instance (#9124)

Long story short: the LeaveStandby event is sent via `tickAfterNext`, so if we tear down the plugin right after calling it (in this case, that means that the very input event that wakes the device up from suspend is one that kills ReaderUI or FileManager), what's in UIManager's task queue isn't the actual function, but the anonymous nextTick wrapper constructed by `tickAfterNext` (c.f., 
https://github.com/koreader/koreader/issues/9112#issuecomment-1133999385).

Tweak `UIManager:tickAfterNext` to return a reference to said wrapper, so that we can store it and unschedule that one, too, in `AutoSuspend:onCloseWidget`.

Fix #9112 (many thanks to [@boredhominid](https://github.com/boredhominid) for his help in finding a repro for this ;)).
Re: #8638, as the extra debugging facilities (i.e., ebb81b9845) added during testing might help pinpoint the root issue for that one, too.

Also includes a minor simplification to `UIManager:_checkTasks`, and various other task queue related codepaths (e.g., `WakeupMgr`) ;).

plugins/autosuspend.koplugin/main.lua

@@ -33,11 +33,12 @@ local AutoSuspend = WidgetContainer:new{
     auto_suspend_timeout_seconds = default_auto_suspend_timeout_seconds,
     auto_standby_timeout_seconds = default_auto_standby_timeout_seconds,
     last_action_time = 0,
-    is_standby_scheduled = false,
+    is_standby_scheduled = nil,
     task = nil,
     standby_task = nil,
     leave_standby_task = nil,
-    going_to_suspend = false,
+    wrapped_leave_standby_task = nil,
+    going_to_suspend = nil,
 }
 
 function AutoSuspend:_enabledStandby()
@@ -135,6 +136,10 @@ function AutoSuspend:init()
 
     if Device:isPocketBook() and not Device:canSuspend() then return end
 
+    -- We only want those to exist as *instance* members
+    self.is_standby_scheduled = false
+    self.going_to_suspend = false
+
     UIManager.event_hook:registerWidget("InputEvent", self)
     -- We need an instance-specific function reference to schedule, because in some rare cases,
     -- we may instantiate a new plugin instance *before* tearing down the old one.
@@ -199,7 +204,14 @@ function AutoSuspend:_unschedule_standby()
     end
 
     -- Make sure we don't trigger a ghost LeaveStandby event...
+    if self.wrapped_leave_standby_task then
+        logger.dbg("AutoSuspend: unschedule leave standby task wrapper")
+        UIManager:unschedule(self.wrapped_leave_standby_task)
+        self.wrapped_leave_standby_task = nil
+    end
+
     if self.leave_standby_task then
+        logger.dbg("AutoSuspend: unschedule leave standby task")
         UIManager:unschedule(self.leave_standby_task)
     end
 end
@@ -326,6 +338,8 @@ end
 
 function AutoSuspend:onLeaveStandby()
     logger.dbg("AutoSuspend: onLeaveStandby")
+    -- If the Event got through, tickAfterNext did its thing, clear the reference to the initial nextTick wrapper...
+    self.wrapped_leave_standby_task = nil
     -- Unschedule suspend and shutdown, as the realtime clock has ticked
     self:_unschedule()
     -- Reschedule suspend and shutdown (we'll recompute the delay based on the last user input, *not* the current time).
@@ -587,7 +601,7 @@ function AutoSuspend:AllowStandbyHandler()
         -- (in case we were woken up by user input, as opposed to an rtc wake alarm)!
         -- (This ensures we'll use an up to date last_action_time, and that it only ever gets updated from *user* input).
         -- NOTE: UIManager consumes scheduled tasks before input events, which is why we can't use nextTick.
-        UIManager:tickAfterNext(self.leave_standby_task)
+        self.wrapped_leave_standby_task = UIManager:tickAfterNext(self.leave_standby_task)
     end
 end