Add flag for supporting visible background users.

The main use case for visible background users is the Automotive
multi-display configuration where a passenger can use a secondary
display while the driver (current user) is using the main display.

Bug: 393987170
Flag: com.android.documentsui.flags.support_visible_background_user
Test: m all_aconfig_declarations
      printflags | grep support_visible_background_user

Change-Id: I5c0a1fcf224eddbab671cf7c9efe87fc3b1e4f47