title: WorkManager.getInstance().cancelAllWorkByTag() not stopping the periodic job
body: <p>for periodic task I am using work manager as:</p>
  
  <pre><code>PeriodicWorkRequest.Builder wifiWorkBuilder =
                              new PeriodicWorkRequest.Builder(FileUpload.class, 15,
                                      TimeUnit.MINUTES)
                                      .setConstraints(new Constraints.Builder().setRequiredNetworkType(NetworkType.METERED).build());
                      PeriodicWorkRequest wifiWork = wifiWorkBuilder.build();
                      WorkManager.getInstance().enqueueUniquePeriodicWork("wifiJob", ExistingPeriodicWorkPolicy.REPLACE, wifiWork);
  </code></pre>
  
  <p>I have one activity in that activity one check box is there if user unselect checkbox then I want to stop this job. So I am stopping job like:</p>
  
  <pre><code>WorkManager.getInstance().cancelAllWorkByTag("wifiJob");
  </code></pre>
  
  <p>But after stopping job also my task is executing. Previously I thought might be next job will be executed and after that it will stop but in last 1 hour it executed 4 times and still job is running in background. What is other way or correct way to stop job. </p>
  
  <p>as per Docs:</p>
  
  <blockquote>
    <p>Cancels all unfinished work with the given tag. Note that cancellation is a best-effort policy and work that is already executing may continue to run.</p>
  </blockquote>
  
  <p>What is the meaning of this -  cancellation is a best-effort policy and work that is already executing may continue to run. So what is the correct way to stop this </p>
  
  <p>I am using version <code>implementation "android.arch.work:work-runtime:1.0.0-alpha08"</code>
  Thanks in advance </p>