Expose VCN APIs for framework code

As the first step of mainlining VCN, VCN will be moved from
framework to its own java_sdk_library and hidden APIs will no longer
be accessible to framework code. This CL exposes these classes,
methods and constants as @SystemApi so that framework code can
continue accessing them.

Note, these APIs are temporarily exposed from framework.jar. Followup
CLs will move them to the framework-connectivity-b.jar

Bug: 376339506
Test: atest FrameworksVcnTests && atest CtsVcnTestCases
Flag: android.net.vcn.mainline_vcn_module_api
API-Coverage-Bug: 376553318
Change-Id: Ibf8a2d32ec3eae73cfb279dde2a0c74e4b4c5b33