Make default namespace handle the 'visible' flag.

'visible' flag was interpreted only the namespaces other than 'default'.
The flag has to be handled without exception once it is set.

Bug: 63411330
Test: Build and boot without errors.
Test: Add `namespace.default.visible = true` to [vendor] process in
      'ld.config.txt' and reboot the device.

Change-Id: Ia363dfadf0200317b875e26bb4b2fae849af1384