audio/avrcp: Always update transport volume regardless of player

audio/avrcp: Always update transport volume regardless of player

`Volume` is a special property that not only exists on players but also
on the transport (see org.bluez.MediaTransport1). A player is not
attached when the controller does not support FEATURE_CATEGORY_1, which
is common on headphones without media browsing capabilities.

On such audio devices (headphones, in-ears and the like) Absolute Volume
is not available unless an external player is registered
(org.bluez.Media1.RegisterPlayer) and the device sends a volume event
back after that to set a2dp->volume in transport.c to a valid value
(causing volume_exists to finally return true).

This [1] mail thread denoting the same issue has a solution to at least
request capabilities from the controller, but the proposed player object
is not created on category 2 devices. Any notifications received on
AVRCP_EVENT_VOLUME_CHANGED (avrcp_volume_changed) that is subsequently
registered, or handling the result of avrcp_set_volume in
avrcp_handle_set_volume will be ignored unless said player is present.

This issue is not addressed by adding a fake player but instead dealing
with the fact that volume is "special" and available on the transport
regardless of the existence of a player. This is confirmed in
avrcp_get_capabilities_resp as well which requires a player to register
any event except AVRCP_EVENT_VOLUME_CHANGED.

The applied solution moves media_transport_update_device_volume out of
the player and into avrcp_volume_changed/avrcp_handle_set_volume where
it is unconditionally called. These functions are the only users of
avrcp_player->set_volume.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>