FileBridge needs to keep strong reference to PFD.

Even though we've grabbed the underlying FD, the PFD could be GC'ed
and when finalized it would end up closing the underlying FD.  This
fix ties the PFD object lifecycle to the returned OutputStream.

Bug: 17183379
Change-Id: Ibee8f4cf78fee357181a250d15f2a653294b2877