[VCN05] Pass request type when requesting network

Currently, ConnectivityService decides the request type by
whether NetworkCapabilities is null when handling request
network. However, to fulfill the need of firing background
request via ConnectivityManager in the follow-up patches,
the request type is needed to pass into ConnectivityService.

This change also make ConnectivityService utilizes the passed
request type.

Test: atest ConnectivityManagerTest#testRequestType
Bug: 175662146
Change-Id: I3bc172bca1217c8020db45057a621d0745d43b3c