Bluetooth: host: Update AVDTP to use new delayable work API

Update AVDTP to use the new delayable work API for the send host timer.
The AVDTP protocol is incomplete, there is no cancelation of the timer
but it is also never started, as there is no users of the internal
functions, and no public functions exists.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>