Fixed null pointer issue.

Summary:
We need to call unbindService on a Context.  In some cases, mAuthActivity
is null.  Since applicationsContext is passed in to the constructor
and never modified, it should not be null.  So we call unbindService
on applicationsContext instead.

Test Plan: Try SSO with pre-1.8 FB app.

Reviewers: vijaye, jacl, caabernathy

Reviewed By: vijaye

CC: selekman, constantin, lshepard

Differential Revision: https://phabricator.fb.com/D432892

Revert Plan: Safe to revert

Task ID: 982981

facebook/src/com/facebook/android/Facebook.java

@@ -582,7 +582,7 @@ public void onServiceDisconnected(ComponentName arg) {
             serviceListener.onError(new Error("Service disconnected"));
             // We returned an error so there's no point in
             // keeping the binding open.
-            mAuthActivity.unbindService(TokenRefreshServiceConnection.this);
+            applicationsContext.unbindService(TokenRefreshServiceConnection.this);
         }
 
         private void refreshToken() {