os: skip Open("/") on Android

It's not supported in an app context:

$ go test -short os
--- FAIL: TestChdirAndGetwd (0.00s)
    os_test.go:1213: Open /: open /: permission denied

Change-Id: I56b951f925a50fd67715ee2f1de64951ee867e91
Reviewed-on: https://go-review.googlesource.com/c/go/+/170946


Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>