Rename some Vpn fields.

This member has an extremely confusing name, and the logic
handling it being convoluted does not help. This change renames
the member and add comments.

Bug: 112341346
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: Icfb6098d1c9e6293078ea7eede3f24a2fabe9d6b
1 file changed