Clean up time zone handling in MtpUtils::parseDateTime.

The intent here is supposed to be to use local time (which mktime(3) does)
unless the string ends 'Z' in which case we're supposed to use UTC. The UTC
equivalent of mktime(3) is timegm(3).

This code has come to my attention in two ways. Externally it was pointed
out that the localtime_r(3) was accessing uninitialized memory leading to
unpredicatable results. Internally, this is the only caller of the
non-standard mktime_tz --- ironically using it in a way that -- apart from
the flakiness caused by the uninitialized variable -- would make it behave
like the standard mktime(3).

Bug: https://code.google.com/p/android/issues/detail?id=185159
Bug: http://b/15765976
Change-Id: I5f015727c4c08a139ef478fafba5811a9758d5df