Fix volume change failure in HDMI-CEC system audio mode.

In order to get volume change event, output device should not be
in fixed volume device. This change ignores CEC releated outputs
if system audio mode is on.

Along with this, fix invalid output from getStreamForDevice
as AudioPolicManager does.

Bug: 17502272
Change-Id: I7d4406caea7fa6617f17969dc61aabebb87ecf7d