Minor improvements to verifyTcpBufferSizeChange.

1. Remove "throws Exception" from the signature so it can be used
   in tests that do not already throw Exception.
2. Update comment to reflect the fact that TCP buffer sizes are
   set on default network switch, not on connect.
3. Move into the method a class-level static constant that is
   only used in that method.

Test: atest ConnectivityServiceTest
Change-Id: Ic2e4fbedc23065efc20e45ea84996b577a8e94b6