This works (by reading /etc/localtime) on NetBSD, but not on Android
since we have no such file. Fix that by using our equivalent system
property instead.
Also s/time zone/timezone/ in documentation and comments. We've always
been inconsistent about this (as is upstream in code comments and
documentation) but it seems especially odd now we expose a _type_ that
spells it "timezone" to talk of "time zone" even as we're describing
that type and its associated functions.
Bug: https://github.com/chronotope/chrono/issues/499
Test: treehugger
Change-Id: I142995a3ab4deff1073a0aa9e63ce8eac850b93d
31fc69f67f