PrintSpooler: Avoid crash in DocumentTransformer shutdown

When DocumentTransformer's background task finishes, onPostExecute tries
to unbind the PdfManipulationService that was previously bound in
transform().  If the background task finishes after the PrintActivity is
already closing, the service may have already been unbound.  Since this
is happening during final cleanup of DocumentTransformer already, the
resulting exception can be suppressed instead of thrown.

Bug: 438886442
Test: Save a 1200 page document to PDF
Flag: EXEMPT bugfix
Change-Id: I535c9c5350ce93adeeccfc6913bee4b6bf58dcfa