Add a settings entry to enable UTC to be selected

There have been numerous complaints that the Android
settings cannot support selection of UTC as the device
time zone.

Previously this was not easily possible because ICU /
CLDR did not make translated strings for UTC available.

After discussion with mark@ and yoshito@ (et al) from
ICU and the import of ICU 59 into Android we can now add
UTC as a settings option.

Much credit goes to to ICU folks for addressing this for
us in ICU 59, and various folks in Android that worked
on the ICU 59 pull (roubert@ and jsauer@ particularly).
This is the culmonination of 12 months work for a one line
change that should make some folks very happy.

Bug: 36957647
Bug: 36337342
Bug: http://unicode.org/cldr/trac/ticket/9943
Test: Manual testing with various apps: calendar / clock
Change-Id: I3901f166f9d496ab83f4584026c663c192b5f91b