[NS B03] Add debug log showing the reassignment

dumpString for a reassignment looks like :

NetworkReassignment :
  Rematched networks : [100 CELLULAR], [101 WIFI]
  6 : 100 → 101
  8 : null → 101

toString looks like :

NetReassign [4 : 100 → 101, 5 : null → 101]

If no changes, then it looks like
NetworkReassignment : no changes

Bug: 113554781
Test: Manual
Change-Id: If9eeadb7ee317dee2d91ca1feca3091ae39e9bae