time: load time zones from the system tzdata file on Android

time: load time zones from the system tzdata file on Android

Android timezones are in a packed format, different from the separate
files of a regular Unix system. This CL contain the necessary code to
parse the packed tzdata file and extract time zones from it. It also
adds a basic test to ensure the new parser works.

Fixes #13581

Change-Id: Idebe73726c3d4c2de89dd6ae1d7d19f975207500
Reviewed-on: https://go-review.googlesource.com/24494
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>