[CLATJ#25] ClatCoordinator: stop clatd process gracefully

Send SIGTERM with timeout first, if fail, SIGKILL.

Bug: 212345928
Test: connect/disconnect to IPv6 only network and ping 8.8.8.8
      repeat 3 times
Change-Id: I5125a3ac7e8ab6d4e8ae7fc7134b8ac8e42ce5dd
1 file changed