Some devices (Bose Mini Soundlink II, Air 1 Plus, ...) don't enable
AVRCP volume control, or fail to enable it before a hardware button is
pressed.  However, these devices appear to enable it, if an AVRCP player
is present.

As a workaround, register a dummy AVRCP player for each adapter. It only
displays the current transport acquisition state as playing/stopped, but
just its presence appears to be enough to make devices behave.

Multiple AVRCP players interfere with each other, as BlueZ uses the one
registered earliest as the default player. So add also a config option
for disabling this. (It's not common to have mpris-proxy etc. running,
so defaulting to true should be OK.)

See #1157