Add VcnContext, Vcn skeletons

This change adds the Vcn class, which represents a single carrier,
single-subscription-group abstraction of the Network.

Additionally, this adds a VCN context, which encapsulates context,
looper and network providers to ensure all instances uses the same
context, looper and network provider.

Bug: 163432852
Test: atest FrameworksVcnTests
Change-Id: I19ecdb4b406102e571029b5520a1ff028bfa1064