os: make UserHomeDir specialize behavior for GOOS=android

Change-Id: I69582662aeee7344226856c24907516ddfc92f60
Reviewed-on: https://go-review.googlesource.com/c/139717
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>

src/os/file.go

@@ -387,16 +387,16 @@ func UserCacheDir() (string, error) {
 // On Windows, it returns the concatenation of %HOMEDRIVE% and %HOMEPATH%.
 // On Plan 9, it returns the $home environment variable.
 func UserHomeDir() string {
-	if runtime.GOOS == "windows" {
+	switch runtime.GOOS {
+	case "windows":
 		return Getenv("HOMEDRIVE") + Getenv("HOMEPATH")
-	}
-	if runtime.GOOS == "plan9" {
+	case "plan9":
 		return Getenv("home")
-	}
-	if runtime.GOOS == "nacl" {
+	case "nacl", "android":
 		return "/"
-	}
+	default:
 		return Getenv("HOME")
+	}
 }
 
 // Chmod changes the mode of the named file to mode.