Bug 1446202 - 1. Return correct locale string to Gecko; r=droeh

Locale.toString() doesn't return the correct locale format that Gecko
expects, so reformat the locale string before returning. (for example,
Locale.toString() returns "en_US_POSIX#Latn" whereas Gecko expects
"en-Latn-US-POSIX"). The incorrect locale format was preventing intl
code (and the datetime box) from initializing.

MozReview-Commit-ID: JmDYkDj31pL

--HG--
extra : rebase_source : 545e8f4e57e1e0086d3afb862c9dfa82ff89345f

mobile/android/geckoview/src/main/java/org/mozilla/gecko/GeckoAppShell.java

@@ -76,6 +76,7 @@ import android.media.AudioManager;
 import android.net.ConnectivityManager;
 import android.net.NetworkInfo;
 import android.net.Uri;
+import android.os.Build;
 import android.os.Bundle;
 import android.os.Environment;
 import android.os.Looper;
@@ -1872,6 +1873,21 @@ public class GeckoAppShell
 
     @WrapForJNI
     public static String getDefaultLocale() {
-        return Locale.getDefault().toString();
+        final Locale locale = Locale.getDefault();
+        if (Build.VERSION.SDK_INT >= 21) {
+            return locale.toLanguageTag();
+        }
+
+        final StringBuilder out = new StringBuilder(locale.getLanguage());
+        final String country = locale.getCountry();
+        final String variant = locale.getVariant();
+        if (!TextUtils.isEmpty(country)) {
+            out.append('-').append(country);
+        }
+        if (!TextUtils.isEmpty(variant)) {
+            out.append('-').append(variant);
+        }
+        // e.g. "en", "en-US", or "en-US-POSIX".
+        return out.toString();
     }
 }