cmd/gomobile: remove unnecessary IsDir check

The check when looking for android platform is unnecessarily stringent
in that the error would be caught by the call to os.Stat, and
it fails when the android platform folder is a symlink,
which is the case when developing on NixOs.

Change-Id: I07a96c969ffa29dea8bf8e353264e8597dcdc550
GitHub-Last-Rev: c684b80
GitHub-Pull-Request: #55
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/267081
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

Author: jmorag

cmd/gomobile/bind_androidapp.go

@@ -393,7 +393,7 @@ func androidAPIPath() (string, error) {
 	var apiVer int
 	for _, fi := range fis {
 		name := fi.Name()
-		if !fi.IsDir() || !strings.HasPrefix(name, "android-") {
+		if !strings.HasPrefix(name, "android-") {
 			continue
 		}
 		n, err := strconv.Atoi(name[len("android-"):])