Replace BaseNetworkObserver with BaseNetdUnsolicitedEventListener

ConnectivityService will be a part of mainline module, to prevent
using @hide API of NetworkManagementService in TestNetworkService,
use function inside NetdUtils instead.

Bug: 170598012
Test: atest FrameworksNetTests CtsNetTestCasesLatestSdk
Change-Id: Id615f7b900d26d0f5887adcd8221dc8bef239923