Fix setTimeZone() and use it properly in RSTest/rstime.

BUG=5470134

The original implementation for rsi_ScriptSetTimeZone() never actually did
anything with the bytes received. This change allows it to safely update the
timezone. RSTest is also updated to call setTimeZone(), so that users in
different timezones can accurately get test results.

Change-Id: I6cb1b3a0c3a417749ba39e0fe09cc9c7ab65c2e7