Cache the most-recently used timezone for mktime_tz and localtime_tz.

Normally, the C library implicitly caches your timezone by virtue
of the fact that the prehistoric API assumes a single timezone for
the entire process.

The unfortunate mktime_tz and localtime_tz extensions work around
this, but represent timezones as strings to their callers, so code
that makes heavy use of these needs a cache to be able to perform
acceptably until it can hopefully one day be rewritten to use
java.util.Calendar or icu4c.

Bug: 8270865
Change-Id: I92e3964e86dc33ceac925f819cc5e26ff4203f50