Loading location data from tzdata files was only supported
from default paths on android. This change enables support on
all OS via the ZONEINFO environment variable and reduces the
amount of android specific code significantly.
Furthermore, unsuccessful calls to LoadLocation now return the
first error encountered, including errors from attempting to
load a location from the source specified by ZONEINFO.
Errors indicating that the source or location was not found are
ignored until all possible sources have been traversed.

Change-Id: I45bc23b92253c9447f12f95f3ca29a7e613ed995
Reviewed-on: https://go-review.googlesource.com/67170Reviewed-by: Ian Lance Taylor <iant@golang.org>