Move PluginShare.backgroundJobs into PluginShare module (#3128)

frontend/pluginshare.lua

@@ -1,3 +1,5 @@
 -- PluginShare is a table for plugins to exchange data between each other. Plugins should maintain
 -- their own protocols.
-return {}
+return {
+    backgroundJobs = {},
+}

plugins/autofrontlight.koplugin/main.lua

@@ -22,8 +22,6 @@ local AutoFrontlight = {
   last_brightness = -1,
 }
 
-PluginShare.backgroundJobs = PluginShare.backgroundJobs or {}
-
 function AutoFrontlight:_schedule(settings_id)
     local enabled = function()
         if not self.enabled then

plugins/backgroundrunner.koplugin/main.lua

@@ -64,8 +64,6 @@ local logger = require("logger")
 -- end_sec: number, the os.time() when the job was stopped.
 -- insert_sec: number, the os.time() when the job was inserted into queue.
 
-PluginShare.backgroundJobs = PluginShare.backgroundJobs or {}
-
 local BackgroundRunner = {
     jobs = PluginShare.backgroundJobs,
 }