os/user: use the stubs fallback for Android

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>

src/os/user/lookup_android.go

@@ -8,15 +8,6 @@ package user
 
 import "errors"
 
-func init() {
-	userImplemented = false
-	groupImplemented = false
-}
-
-func current() (*User, error) {
-	return nil, errors.New("user: Current not implemented on android")
-}
-
 func lookupUser(string) (*User, error) {
 	return nil, errors.New("user: Lookup not implemented on android")
 }
@@ -32,7 +23,3 @@ func lookupGroup(string) (*Group, error) {
 func lookupGroupId(string) (*Group, error) {
 	return nil, errors.New("user: LookupGroupId not implemented on android")
 }
-
-func listGroups(*User) ([]string, error) {
-	return nil, errors.New("user: GroupIds not implemented on android")
-}

src/os/user/lookup_stubs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !cgo,!windows,!plan9,!android
+// +build !cgo,!windows,!plan9 android
 
 package user
 
@@ -46,6 +46,9 @@ func current() (*User, error) {
 }
 
 func listGroups(*User) ([]string, error) {
+	if runtime.GOOS == "android" {
+		return nil, errors.New("user: GroupIds not implemented on Android")
+	}
 	return nil, errors.New("user: GroupIds requires cgo")
 }