Don't rely on fallback time zone names starting "GMT".

I can't think of a way to say what I actually want to say here, so let's
just wait for icu4c 50 and hope we can rewrite all this code with the new
TimeZoneNames API. Until then, various locales -- those whose gmtFormat
doesn't start "GMT" -- still have the bugs that the previous change fixes
for the majority of locales.

Also switch from "Samoa Summer Time" to "Samoa Daylight Time" because that's
what CLDR is proposing to go with.

Change-Id: I47e2fc413ccde6af59f51e5ec79e664a1dc8be82