examples/ubuntu: switch to libevent network provider

- remove network provider impl from network_manager
- rename network_manager to wifi_manager

Bug: 24466635
Change-Id: I23845a59f0602c3995f5ece07a31d4a81f23b224
Reviewed-on: https://weave-review.googlesource.com/1331
Reviewed-by: Vitaly Buka <vitalybuka@google.com>