PM: Optimize task queue handling around standby (#10203)

Instead of firing on(Enter|Leave)Standby Events, and having every other piece of code that might care about that handle re-scheduling their stuff themselves; simply make the standby implementation (i.e., AutoSuspend's) shift the whole task queue by the amount of time spent in standby to re-sync everything automatically.

(This is necessary in the first place because Linux, as the task queue ticks in CLOCK_MONOTONIC, which does *not* tick during suspend/standby; while we expect most of the tasks scheduled to actually reflect real world clock delays).

frontend/apps/reader/modules/readerfooter.lua

@@ -2471,21 +2471,12 @@ function ReaderFooter:onOutOfScreenSaver()
     self:rescheduleFooterAutoRefreshIfNeeded()
 end
 
-function ReaderFooter:onLeaveStandby()
-    self:maybeUpdateFooter()
-    self:rescheduleFooterAutoRefreshIfNeeded()
-end
-
 function ReaderFooter:onSuspend()
     self:unscheduleFooterAutoRefresh()
     -- Reset the initial marker
     self.progress_bar.inital_percentage = nil
 end
 
-function ReaderFooter:onEnterStandby()
-    self:unscheduleFooterAutoRefresh()
-end
-
 function ReaderFooter:onCloseDocument()
     self:unscheduleFooterAutoRefresh()
 end