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

tests/Android.mk

@@ -32,6 +32,7 @@ benchmark_src_files = \
     benchmark_main.cpp \
     math_benchmark.cpp \
     string_benchmark.cpp \
+    time_benchmark.cpp \
 
 # Build benchmarks for the device (with bionic's .so). Run with:
 #   adb shell bionic-benchmarks