[NS A38] Fill the initial reassignment

This is more expensive for now but it will allow subsequent patches
to read relevant state from the reassignment instead of from the
global state, which will stop the computation of the reassignment
from reading state that is mutated by the same loop. Eventually
this lets us completely split the computation from the side effects.
The ugly parts of this patch will be cleaned up later as a result,
namely in patches [NS B04] and [NS B05].

Test: ConnectivityServiceTest
Change-Id: I271e7f4d4bc81493c1ea212025b7130619592a8a