Fetch roots from all users and filter them by action

If we enable the compile-time feature flag and open pick activity,
we can see "duplicate" roots on a device with managed profiles.
Those roots will navigate to the directory of the respective user.

Bug: 148264331
Test: atest DocumentsUIGoogleTests:com.android.documentsui.UserIdManagerTest
Test: atest DocumentsUIGoogleTests
Test: manual

Change-Id: Ie2d87009e96d6ffd2c634cb10945239de1d96720