Using the stubs, user.Current will no longer fail on Android, fixing
the os/exec.TestCredentialNoSetGroups test.

Change-Id: I8b9842aa6704c0cde383c549a614bab0a0ed7695
Reviewed-on: https://go-review.googlesource.com/37765
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>