Do the refactor in RecentsLoader and support global search

1. Do refactor in RecentsLoader.
2. Create MultiRootDocumentsLoader extracted from RecentsLoader.
3. Support only query Root.FLAG_LOCAL_ONLY roots in RecentsLoader.
4. Add GlobalSearchLoader to support global search in Recents root.

Bug: 111695441
Fix: 119109832
Test: atest DocumentsUITests
Change-Id: I19df6d58a7d1df042eef925575dc8d56cf81fa09