Fix potential exceptions in FdEventReader users

In particular,
 - Fix a possible NPE In IpNeighborMonitor, if a NUD_FAILED is received
   for some neighbor before all other neighbors have received
   RTM_NEWNEIGH.
 - Add try / catch in IpNeighborMonitor, DhcpPacketListener,
   ConnectivityPacketTracker in case some unexpected exception is thrown
   while processing packets.
 - Add Log.wtf logging in FdEventsReader in case any such exception is
   missed.

Bug: 152842850
Test: atest NetworkStackTests
Original-Change: https://android-review.googlesource.com/1274609
Merged-In: Ia03fec358e98b3f218220e70fc3265e14ca15f78
Change-Id: Ia03fec358e98b3f218220e70fc3265e14ca15f78