Expose NetworkSpecifier methods as SystemApi

Support for Wi-Fi mainline module.

Bug: 135998869
Test: build and make system-api-stubs-docs-update-current-api
Test: atest FrameworksNetTests
Change-Id: I88f00ca4f4cd51c71f7668b34f101fe7d1332c5c
Merged-In: Id705ee0f2b5c464f6d8ccc91f20dcb39dcf6f2b8