Fix ANR in print spooler.

Saving to PDF a subset of the pages requires trimming the
undesired pages and when this is done writing the ready doc
to a URI. To write the file to URI we have to obtain it but
it is aquired by the shredding task and never released. Now
we are releasing the document immediately after trimming it.

bug:17631301

Change-Id: I4db7966c65c75f0f14c3cb52fd83b4d8fd5a4687