Update getDefaultRoot rule

When user in ejectable root then eject the root, is will jump to
Downloads root or Recents root. But in document tree case, it
should jump to storage root rahter than above both.
Fix the rule and move this logic to ProvidersAccess for testing.

Fix: 145186010
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: I3d72da8a3cd1b0e15040ab145216c14519bc642f