SyncAdapter, SubjectAdder: Replace use of AsyncTask with Runnable and Thread.

Because older (API <15) versions of Android don't seem to like us using AsyncTask in
the SyncAdapter (which doesn't have the UI thread), giving us this ExceptionInInitializerError:
"Can't create handler inside thread that has not called Looper.prepare()".
HttpUtils.parseGetFileResponseContent(): Close the ParcelFileDescriptor.

To avoid leaking it. Discovered by StrictMode (see BaseActivity).