Fix TimeZone.getAvailableIDs(int).

This was broken by the removal of the pre-computed raw offsets from
the tzdata file. I think that's still the direction we want to go (with
us hopefully using more of icu4j at some point, and eventually relying
solely on the icu time zone data), so this patch adds code to lazily
evaluate all the offsets by instantiating all the time zones.

(cherry-pick of 065d7764.)

Bug: 16947622
Change-Id: I6d1dfe5ee6c99338f9807c3af5b6f04539c256c3