title: Cannot invoke observe on a background thread
body: <p>I just upgraded to <code>AndroidX</code> and after doing that all my request. I make in background thread do not run but come up with this error.</p>
  
  <pre><code>private void getContactsList() {
      Task.callInBackground((Callable&lt;Void&gt;) () -&gt; { 
       mainActivityViewModel.geContacts(contactListRequest).observe(MainActivity.this, new Observer&lt;ContactListResponse&gt;() {
              @Override
              public void onChanged(@Nullable ContactListResponse contactListResponse) {
                  if(contactListRequest != null){
                      System.out.println("Successful");
                  }
              }
          });
          return null;
      }).continueWith((Continuation&lt;Void, Void&gt;) task -&gt; {
          if (task.isFaulted()) {
              Log.e(TAG, "find failed", task.getError());
          }
          return null;
      });
  }
  
  
  
  java.lang.IllegalStateException: Cannot invoke observe on a background thread
      at androidx.lifecycle.LiveData.assertMainThread(LiveData.java:443)
      at androidx.lifecycle.LiveData.observe(LiveData.java:171)
      at com.qucoon.rubies.MainActivity.lambda$getContactsList$12(MainActivity.java:887)
      at com.qucoon.rubies.-$$Lambda$MainActivity$qPAxeGqyWT-wT3M7e8stM1rX2gQ.call(lambda)
      at bolts.Task$4.run(Task.java:357)
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
      at java.lang.Thread.run(Thread.java:818)
  </code></pre>