Improve reliability of soundbar volume UI on TVs.

Currently, volume behavior can switch from adjust-only AVB to full
volume behavior multiple times shortly after plugging in a soundbar.
This causes volume UI to stop working intermittently, and adds noise
to volume behavior listeners.

This CL makes two changes to make this less likely:
1. When switching from adjust-only AVB to regular AVB, don't switch to
   full volume while waiting for the change to take place. Staying with
   adjust-only AVB is fine because it provides a subset of the features
   of regular AVB.
2. When <SAVL> support becomes unknown, don't switch to full volume
   behavior from adjust-only absolute volume behavior. It's
   unnecessary because adjust-only absolute volume behavior may be used
   regardless of <SAVL> support.

Test: manual with falcon and atom
Bug: 282012994
Change-Id: Ibf99fb67788441d5257c63b64a4b6683c9336798