Fix TimeZoneTest#testPreHistoricOffsets CTS failures.

This test was more complicated than necessary, and now we have better
historical data for Asia/Saigon it's started to fail. I've removed that
part of the test (because there are now a lot more transitions) and rely
solely on Africa/Bissau. We really only need one transition so we can
test two times: one before the first transition and one after the last
transition.

Bug: 18285300
Change-Id: Ic820e88b319095b89779d271f45f5cb1c90173da
(cherry picked from commit d05995d8)