Convert VcnNetworkProvider to use NetworkOffers

This change brings VcnNetworkProvider to full functionality in the new
NetworkProvider paradigm, where NetworkProviders offer networks, and are
notified based on NetworkOfferCallbacks.

Bug: 185204197
Test: atest FrameworksVcnTests
Change-Id: I88c69c0be9f6fd81839fb1595ed00341001694a5