DocumentsUI: Add new private intent action to select copy destination.

The CL adds INTENT_ACTION_OPEN_COPY_DESTINATION, and implements the behavior to
respond the intent action.

The directory picker:
 * filter out read only roots
 * return document URI instead of document tree URI
for the intent action.

BUG=b:20146034
Change-Id: I96c565100c3cdaa57cc7abd8cf6d895d148941a7
diff --git a/src/com/android/documentsui/DirectoryFragment.java b/src/com/android/documentsui/DirectoryFragment.java
index 4cf0180..61fcad2 100644
--- a/src/com/android/documentsui/DirectoryFragment.java
+++ b/src/com/android/documentsui/DirectoryFragment.java
@@ -682,7 +682,11 @@
 
         // Pop up a dialog to pick a destination.  This is inadequate but works for now.
         // TODO: Implement a picker that is to spec.
-        Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
+        final Intent intent = new Intent(
+                BaseActivity.ACTION_OPEN_COPY_DESTINATION_STRING,
+                Uri.EMPTY,
+                getActivity(),
+                DocumentsActivity.class);
         startActivityForResult(intent, REQUEST_COPY_DESTINATION);
     }