os: no /tmp on android

LGTM=minux, bradfitz
R=golang-codereviews, minux, bradfitz
CC=golang-codereviews
https://golang.org/cl/104650043

src/pkg/os/file_unix.go

@@ -308,7 +308,11 @@ func basename(name string) string {
 func TempDir() string {
 	dir := Getenv("TMPDIR")
 	if dir == "" {
+		if runtime.GOOS == "android" {
+			dir = "/data/local/tmp"
+		} else {
 			dir = "/tmp"
 		}
+	}
 	return dir
 }