Make HttpCookie.header private field

- Android has a local patch to make the field public, but hidden
- The patch was introduced to allow access of the field from
  sun.net.www.protocol.http.HttpURLConnection. The class has been
  removed, and we don't need the field public

Bug: 62527337
Test: Test: cts-tradefed run cts-dev --module CtsLibcoreOjTestCases
Change-Id: Ie254865302c6bfc2b4b76dcf85c7aec3ad2d7476
1 file changed