Move thread_util to libnetdutils

This change comprises:

[1] Move thread_util.h to libnetdutils to ease the cleanup of the
    include path system/netd/server for libnetd_resolv.

[2] Rename thread_util.h to ThreadUtil.h.

[3] Add the test to ThreadUtilTest.

Bug: 128662167
Test: system/netd/tests/runtests.sh passed
Change-Id: Ia8e9b196fbe41a4bd1edc7592badaacf57165988