Move selection to selection pkg, make activity scoped.

Move all other selection related classes to selection pkg.

Change-Id: I57a3964fada55b0f4d073f05a7833455235221b9
diff --git a/src/com/android/documentsui/dirlist/Model.java b/src/com/android/documentsui/dirlist/Model.java
index 6c08859..016346a 100644
--- a/src/com/android/documentsui/dirlist/Model.java
+++ b/src/com/android/documentsui/dirlist/Model.java
@@ -33,8 +33,8 @@
 import com.android.documentsui.DirectoryResult;
 import com.android.documentsui.base.DocumentInfo;
 import com.android.documentsui.base.EventListener;
-import com.android.documentsui.dirlist.MultiSelectManager.Selection;
 import com.android.documentsui.roots.RootCursorWrapper;
+import com.android.documentsui.selection.Selection;
 
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;