Refactor LoadRootTask for backup behavior when root doesn't exist.

When root doesn't exist then run LoadRootTask, it will always finish
activity directly. It is very bad user experience.
Refactor LoadRootTask and handle callback on ActionHandler.

Fix: 137053586
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: Ib6536f2f7678f1e9aa0c29a65433e78cab86082f