Add MOUNT_FLAG_VISIBLE to visible stub volumes

The visibility setting of a removable device shared with Chrome OS is
passed via Disk flags. When a device (stub volume) is marked as visible,
we should set the MOUNT_FLAG_VISIBLE flag to it.

Bug: 123377807
Bug: 142684891
Bug: 132796154
Test: Toggle the visibility setting of a removable device in the Chrome
Test: OS Settings app > Confirm that the device is visible to Android
Test: apps only when it is marked as visible.
Test: Tested in R

Change-Id: I19289596345690b1802738122fe274eeb41f9361