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

(cherry picked from commit d05995d8c7488044352343d5125698795032b937)

Change-Id: Ibdf8f1dde02755bf71b7114b22078698e06a1c00
1 file changed