Fix issue 2909189: System property ro.audio.silent no longer mutes system.

Fixed regression introduced by commit 2a6b80bc65c4782b5a7168b300e1dc5ec9f617ee:
master mute was not working if no effect chains were present on session 0.

Change-Id: I66d107e045d159cb94d29c7476fa1e12d92f2ae7