audio service: make dock audio is off by default

It is better to turn dock audio off by default if the user
has not yet explicitly turned it on from the settings menu.
There is a discoverability issue with current implementation
as we do not launch the settings menu at first dock insertion.
Having audio routed to the dock by default would make the tablet
silent for media content when docked and users will not be directed
to the settings menu.

As the dock audio enable setting is for low end docks only
(analog connection and no jack detection) disabling audio for low end docks
by default does not impact manta and prime docks.

Bug 7302106.

Change-Id: I571f361d41940cfb811f038442ae9ba9c6361d1d