Make TimestampTest non-dependant on device's time zone.

testGetMonth and testGetYear explicitly rely that current device's
time zone is within small range around unix epoch. But time zones
with big enough positive offsets like Pacific/Fiji break that
expectation. Hence setting time zone to GMT as these values expect.

Fix: 258160206
Test: passes when time zone set Pacific/Fiji (test fails w/o this
Test: patch)
Change-Id: I04b2ecd33f8e5c74ff9fff40450aaa2bb7868769