Tame BackgroundRunner: stop running when no more job (#6605)

A BackgroundRunner plugin instance will stop running
(rescheduling a check every 2 seconds) when there is no
(or no more) job to run.
Clients of this service now have to emit an event after
adding a job into PluginShare.backgroundJobs, so an
already loaded but stopped BackgroundRunner can notice
it and start running again.

spec/unit/background_task_plugin_spec.lua

@@ -4,12 +4,19 @@ describe("BackgroundTaskPlugin", function()
     local MockTime = require("mock_time")
     local UIManager = require("ui/uimanager")
 
+    local BackgroundTaskPlugin_schedule_orig = BackgroundTaskPlugin._schedule
     setup(function()
         MockTime:install()
         local Device = require("device")
         Device.input.waitEvent = function() end
         UIManager._run_forever = true
         requireBackgroundRunner()
+        -- Monkey patch this method to notify BackgroundRunner
+        -- as it is not accessible to UIManager in these tests
+        BackgroundTaskPlugin._schedule = function(...)
+            BackgroundTaskPlugin_schedule_orig(...)
+            notifyBackgroundJobsUpdated()
+        end
     end)
 
     teardown(function()
@@ -17,6 +24,7 @@ describe("BackgroundTaskPlugin", function()
         package.unloadAll()
         require("document/canvascontext"):init(require("device"))
         stopBackgroundRunner()
+        BackgroundTaskPlugin._schedule = BackgroundTaskPlugin_schedule_orig
     end)
 
     local createTestPlugin = function(executable)