Add API for apps to check if they are the network owner

This CL extends NetworkCapabilities#mEstablishingVpnAppUid
to the network owner app UID and introduces a new public API
to get this owner app's UID.

Bug: 142072839
Test: atest FrameworksNetTests
Change-Id: Id83cdea62b89b586aff74e51e3fee60e53d37d4c