switch default led color to blue

Fixes #3115
Closes #3116

// FREEBIE

res/xml/preferences_notifications.xml

@@ -21,7 +21,7 @@
 
     <ListPreference
         android:key="pref_led_color"
-        android:defaultValue="green"
+        android:defaultValue="blue"
         android:title="@string/preferences__led_color"
         android:dependency="pref_key_enable_notifications"
         android:entries="@array/pref_led_color_entries"

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -403,7 +403,7 @@ public class TextSecurePreferences {
   }
 
   public static String getNotificationLedColor(Context context) {
-    return getStringPreference(context, LED_COLOR_PREF, "green");
+    return getStringPreference(context, LED_COLOR_PREF, "blue");
   }
 
   public static String getNotificationLedPattern(Context context) {