AudioTrack: fix output flags update

Fix regression introduced by commit c2b0c7a18c6c97a4086956ac95f06addd0f3558f
that caused output flags modifications made in AudioTrack::set() to be
dropped wihtout being saved for further use by createTrack_l().

Bug: 213437414
Test: Play Youtube Music and verify use of deep buffer
Change-Id: I3c6c407fa4408c528a68a3d9736690d563bbafc3