Disable eject button for StubVolumes

StubVolumes are managed from outside Android (e.g. Chrome OS). So, they
should not be unmounted via the eject button in DocumentsUI.

Bug: 187111763
Test: (Tested in R) Manually check and confirm that the eject button is
      not shown for mounted StubVolumes.

Change-Id: Ie7c6d1ec846b20694d98f4cb34099ad5dfcf6478
1 file changed