GsmSST: Disregard any invalid verbose values for timezones in NITZ requests

And assume anything resulting in "GMT" is invalid, since that's the
default value returned by libcore

Change-Id: I361650400e71976d8327ee3929947ca26a9eff57
(cherry picked from commit f440bcd71fe735872b7e4c7ecd557503a79df756)