Revert "Extract network observer code from NetworkManagementService."

This reverts commit d8bc829a85c9c9bb390e18b59ef1455b6d0b94ec.

Reason for revert: Refactoring was unnecessary
(Cherry-pick of aosp/888319 with conflicts with go/ag/4230697)

Test: m
Change-Id: Ib4e7da3274e49f25676505a08e6ce7e7b0dfdb8d
2 files changed