Timer: Don't run new task at old timepoint

* Fix a race condition that the task is just finished but IsScheduled
  is true
* Fix a bug where cancelling the old task will make new task run at
  the previous task's scheduled timepoint
* Protect racing read/write to message_loop_thread_ in RunTask() and
  Cancel() by storing a local reference to message_loop_thread_ in
  RunTask()

Bug: 116081383
Test: Run bluetooth_test_common.TimerTest
      reschedule_task_dont_invoke_new_task_early
      schedule_task
      for multiple times
Change-Id: I737a864e1b061adbcd41245711e0d3e8adf448d4