Implement basic VcnConfig and VcnGatewayConnectionConfig

This change adds some of the basic fields in VcnConfig and
VcnGatewayConnectionConfig, plus adds a persistability layer to ensure
all VcnConfig(s) are disk-stable.

Bug: 163602123
Bug: 163594033
Test: New tests added, passing
Change-Id: I2e632532809e7768b284be376f2b0a77f634fef5