Prevent a 4-way binder interlock leading to a leak.

    System server             |        NetworkStack
                              |
NetworkMonitorCallbacks  ←----|--- NetworkMonitorCallbacks$Stub$Proxy
         ↓                    |           ↑
  NetworkAgentInfo            |      NetworkMonitor
         ↓                    |           ↑
NetworkMonitor$Stub$Proxy ----|---→ NetworkMonitorImpl

Bug: b/133174607
Test: Manual. The simplest artifact is observed by watching the output of
adb shell dumpsys meminfo -d com.android.networkstack | grep 'Proxy Binders'
while connecting and disconnecting multiple times to any network.
This will display the number of binder proxies. Before this, the binder
proxy count increases by 1 with each connection and never goes down (there
is some noise, as proxy objects are sometimes created for other reasons,
and get GC'd eventually). After this, the binder proxy count is always
eventually stable at 27 + connected network count.

See the bug for the complete analysis.

Change-Id: Ide2428dab3fcd6d7cd00aa2a9fd99d6c99b815a4