Select a system audio route only when needed

When system audio routes are changd, updateAudioRoutes is called, and
it selects a system audio route even when a none system route was
seleted.
That makes the selected route be unselected and this CL fixes it.

Bug: 65403568
Test: manual tested that the issue is fixed
Change-Id: I5df9d3e5f68cfd40fdb625937f9a8e16f4308e16