Bluetooth: Mesh: Call friendship callbacks at the end

If a user does some heavy operations in friendship callbacks
unintentionally, this may break friendship communication as timers will
be scheduled or messages will be queued only after code execution comes
back from these callbacks. To avoid this issue, call callbacks only after
required operations are done.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>