[NS A04] Store changes in rematchNetworkAndRequests in a map

This is a preliminary change to move the code that chooses what
network gets assigned to what request out of ConnectivityService.

By storing the list of changes first, it becomes possible to move
the changes with side-effects to a later part of the code, after
the decisions have been made. This move is implemented in a later
change.

Bug: 113554781
Test: ConnectivityServiceTest
Change-Id: I60fe318a8eabf13d1c07b144367ca92cf07e734e