Fix "invalid device combination" in system audio mode.

HDMI-CEC system audio mode removes speaker from output
list. It also has multiple outputs by connected outputs.
However, getDeviceForVolume() doesn't cover multiple
output with no speaker or bluetooth case.
This change picks one of system audio outputs in order.

Bug: 17400026
Change-Id: I102fd0052c9c4c9543f9e8f3b16ec6815d7485d8