Expose Basic VCN API surface

This change exposes the Basics of the VCN API surface, including:
- VCN Configurations
- VCN Gateway Configuration superclass (Builder in subclasses)
- VcnManger with configuration management APIs

Bug: 168833866
Test: atest FrameworksVcnTests
Test: atest android.net.vcn
Change-Id: Ia1b3597d903dd84267f66546a90437f9ac45b2de