Re-introduce private APIs to initialise the filesystem

[oparoz]

From @oparoz on September 30, 2015 20:58

It's difficult to assess whether or not I should go back to using private APIs. In my tests with corrupt files, Gallery displays an icon, as expected, but others may get an empty gallery because of a fatal error caused by a file which can't be read.

The problem comes from $rootFolder->getUserFolder($userId) which apparently does not setup the view properly.
owncloud/core#17196

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Copied from original issue: owncloud/gallery#396