[NS A44 2/2] Apply requests after all networks rematching is computed

This patch finally separates completely computing the rematch
from all the side effects. A collateral effect of this is to
compute correctly the background network state in
rematchNetworkAndRequests, which compensates the breakage
from the previous patch.

Bug: 113554781
Test: ConnectivityServiceTest
Change-Id: I998c729c385940168fcd6ba3f2e01911f1844ce1