Update testShouldShowBadge_returnFalse_onVisibleBackgroundUser

The test was passing regardless of the support_visible_background_user
flag due to an incorrect test setup. Now it would fail if it ran on a
device with the flag disabled (which should not happen with the
@RequiresFlagsEnabled annotation).

Bug: 393987170
Flag: com.android.documentsui.flags.support_visible_background_user
Test: atest DocumentsUIUnitTests:IconHelperTest
Change-Id: I2ca4077c8fe7e8dbd48dcb5fc62939994a6ac9ef