Prep VCN files for exposing APIs

Add @hide to methods to clearly show what is being exposed and cleanup
some visibilty/final modifiers.

Bug: 168833866
Test: atest FrameworksVcnTests
Test: atest android.net.vcn
Change-Id: I8e03133053b59fd5b8df640e4b22761f572f9548
4 files changed