Get rid of NetworkMonitorBase helper class.

All it provides is a method to call a signal on the network thread,
so it's not worth the added complexity. Implementations of
NetworkMonitorInterface must hop to the network thread anyway to
guard their members.

Also added some thread annotations to AndroidNetworkMonitor.

Bug: webrtc:9883
Change-Id: I64bb82ea593433f3a52871dbb75eb2ac4f47d69c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181420
Commit-Queue: Taylor <deadbeef@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32087}

sdk/objc/native/src/objc_network_monitor.h

@@ -42,10 +42,6 @@ class ObjCNetworkMonitor : public rtc::NetworkMonitorInterface,
   void Start() override;
   void Stop() override;
 
-  // TODO(deadbeef): Remove this once it's been removed from
-  // NetworkMonitorInterface.
-  void OnNetworksChanged() override {}
-
   rtc::AdapterType GetAdapterType(const std::string& interface_name) override;
   rtc::AdapterType GetVpnUnderlyingAdapterType(
       const std::string& interface_name) override;