[VCN04] Add Subscription Id set into NetworkCapabilities

This is a generic way to request networks that has different
subId but belongs to the same carrier. For example, cellular
networks with different SIM card, or carrier Wifi that
provided by the operator.

Test: atest NetworkCapabilitiesTest#testSubIds
Test: m doc-comment-check-docs -j
Test: atest CtsNetTestCases
Bug: 175662146

Change-Id: Ifca766f5acc73c285948d6251ec31506d9bb0bcb