Fix behavior when receive ACTION_VIEW_DOWNLOADS intent

On previous CL, recent root becomes the default root in browse
mode, so ACTION_VIEW_DOWNLOADS will also launch to recent root.
Add a new function to ensure it will launch to download root instead
of recent root or other default root OEM set.

Fix: 126237300
Test: manual
Test: atest DocumentsUITests
Change-Id: I9af59861ef1091e3184f1fb04dbbe18d00d235f3