Fix issue with deferred start of the background loader.

- Previously we were (incorrectly) assuming that the creation of the
  BackgroundTaskLoader would be followed synchronously by a call to start()
  the background loader itself.  However, now that we post the start of
  the loader, the run() call can be made before the start() call.

Bug: 38310660
Test: This is an intermittent bug, hard to repro.
Change-Id: I4dfd18b8c3c127429c790177e98ca41ec70f493d