Initial implementation of OPEN_EXTERNAL_DIRECTORY.

It opens a an alert dialog that tells user application X is requesting
access to folder Y on volume Z; if user accepts, it returns the proper
URI, otherwise it returns RESULT_CANCELED.

The requested path must satisfy the following requirements:

1.It's directory (getName()) must be one of the pre-definied folders on
  Environment.
2.It's root (getParent()) must be a valid external storage root.

In order to satisfy #2, DocumentsUI calls the ExternalStorageProvider,
through a custom call() implementation.

BUG:  23011462
Change-Id: I8e491e82ae320df88f7b8923286c4c1ad050ae80
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index ba215bb..c776ef8 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -402,7 +402,7 @@
      * type.
      */
     public static String DIRECTORY_PODCASTS = "Podcasts";
-    
+
     /**
      * Standard directory in which to place any audio files that should be
      * in the list of ringtones that the user can select (not as regular
@@ -414,7 +414,7 @@
      * type.
      */
     public static String DIRECTORY_RINGTONES = "Ringtones";
-    
+
     /**
      * Standard directory in which to place any audio files that should be
      * in the list of alarms that the user can select (not as regular
@@ -426,7 +426,7 @@
      * type.
      */
     public static String DIRECTORY_ALARMS = "Alarms";
-    
+
     /**
      * Standard directory in which to place any audio files that should be
      * in the list of notifications that the user can select (not as regular
@@ -438,7 +438,7 @@
      * type.
      */
     public static String DIRECTORY_NOTIFICATIONS = "Notifications";
-    
+
     /**
      * Standard directory in which to place pictures that are available to
      * the user.  Note that this is primarily a convention for the top-level
@@ -446,7 +446,7 @@
      * in any directory.
      */
     public static String DIRECTORY_PICTURES = "Pictures";
-    
+
     /**
      * Standard directory in which to place movies that are available to
      * the user.  Note that this is primarily a convention for the top-level
@@ -454,7 +454,7 @@
      * in any directory.
      */
     public static String DIRECTORY_MOVIES = "Movies";
-    
+
     /**
      * Standard directory in which to place files that have been downloaded by
      * the user.  Note that this is primarily a convention for the top-level
@@ -464,7 +464,7 @@
      * backwards compatibility reasons.
      */
     public static String DIRECTORY_DOWNLOADS = "Download";
-    
+
     /**
      * The traditional location for pictures and videos when mounting the
      * device as a camera.  Note that this is primarily a convention for the
@@ -496,7 +496,7 @@
      * </ul>
      * @hide
      */
-    public static final String[] STANDARD_DIRECTORIES = {
+    private static final String[] STANDARD_DIRECTORIES = {
             DIRECTORY_MUSIC,
             DIRECTORY_PODCASTS,
             DIRECTORY_RINGTONES,
@@ -510,6 +510,18 @@
     };
 
     /**
+     * @hide
+     */
+    public static boolean isStandardDirectory(String dir) {
+        for (String valid : STANDARD_DIRECTORIES) {
+            if (valid.equals(dir)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
      * Get a top-level shared/external storage directory for placing files of a
      * particular type. This is where the user will typically place and manage
      * their own files, so you should be careful about what you put here to
@@ -559,7 +571,7 @@
         throwIfUserRequired();
         return sCurrentUser.buildExternalStorageAppDataDirs(packageName);
     }
-    
+
     /**
      * Generates the raw path to an application's media
      * @hide
@@ -568,7 +580,7 @@
         throwIfUserRequired();
         return sCurrentUser.buildExternalStorageAppMediaDirs(packageName);
     }
-    
+
     /**
      * Generates the raw path to an application's OBB files
      * @hide
@@ -577,7 +589,7 @@
         throwIfUserRequired();
         return sCurrentUser.buildExternalStorageAppObbDirs(packageName);
     }
-    
+
     /**
      * Generates the path to an application's files.
      * @hide
@@ -595,7 +607,7 @@
         throwIfUserRequired();
         return sCurrentUser.buildExternalStorageAppCacheDirs(packageName);
     }
-    
+
     /**
      * Return the download/cache content directory.
      */