[NS A37] Don't reassign requests multiple times

This is an optimization that skips doing intermediate assignments
of networks to requests that will undergo multiple changes during
the recomputation.
It happens to fix a bug where some of these intermediate states
used to have a visible, transient side effect.

Bug: 113554781
Test: ConnectivityServiceTest
Change-Id: I7af3728152a1cf7571de67f394088a5970ee3c1e