Refactor to IpNeighborMonitor and single-threaded semantics

IpReachabilityMonitor (and IpNeighborMonitor) are only accessed from
the IpManager StateMachine's thread. Consequently lots of locking can
now be removed.

Additionally:
    - rename BlockingSocketReader to PacketReader
    - incorporate IpReachabilityMonitor output in dump()

Test: as follows
    - runtest frameworks-net passes
    - "adb shell ip neigh change <address> dev wlan0 nud failed"
      triggers wifi to disconnect
Bug: 62476366
Bug: 67013397
Change-Id: I18aca29ae0019a72a7e559c2832e0d9b0e33d81e