Fix additional thread safety issue in timezone code

This is an update to the previous fix at
 https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1602037

It takes care of threading issue in initDefault in timezone.cpp as well.

See
https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8913891060019594592/+/steps/Check/0/logs/DateCache.AdoptDefaultMixed/0

Upstream bug: https://unicode-org.atlassian.net/browse/ICU-20595
upstream PR: https://github.com/unicode-org/icu/pull/649/

Bug: chromium:950851
Test: v8 unittests - DateCache:AdoptDefault*
Change-Id: Ifc215e02992050af5c55e1cc0208cff61112c9ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/1604257
Reviewed-by: Jungshik Shin <jshin@chromium.org>