go.mobile/app: do not log info android logcat already includes

i.e. all of log.LstdFlags

LGTM=hyangah
R=golang-codereviews, hyangah
CC=golang-codereviews
https://golang.org/cl/138310043

app/stdio_android.go

@@ -58,6 +58,8 @@ func lineLog(f *os.File, priority C.int) {
 
 func init() {
 	log.SetOutput(infoWriter{})
+	// android logcat includes all of log.LstdFlags
+	log.SetFlags(log.Flags() &^ log.LstdFlags)
 
 	r, w, err := os.Pipe()
 	if err != nil {