Bluetooth: Mesh: Fix registering GATT Proxy service after reboot

After commit https://github.com/zephyrproject-rtos/zephyr/commit/3486f133e9dcfc48e0291b9a31fdbdc9af2f5f09


GATT Proxy service registration is broken because it is registered from
settings_load() in mesh_commit(). Because mesh_commit() is called before
sc_commit() is called in gatt.c, bt_gatt_service_register() can't be
called yet.

Use k_work to postpone the service registration.

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