App manager not respecting writable flag when folder writable on disk

[PVince81]

Steps

1. Setup ownCloud 10.2.0 RC1 enterprise complete which comes with files_pdfviewer 0.10.0 bundled
2. Check that two folders "apps" and "apps-external" exist.
3. Check that config.php states that "apps" is not writable, but leave the folder writable on disk
4. Use market app to update files_pdfviewer to 0.11.0

Expected result

Updated files_pdfviewer lands in "apps-external".

Actual result

Updated files_pdfviewer lands in "apps" despite the folder being marked non-writable in config.

Version

ownCloud enterprise complete 10.2.0 RC1

I suspect that the code is only checking the permissions on disk, not the actual writable flag.
This defeats the purpose of the "apps-external" folder which is supposed to be the recipient for any app updates.

FYI @mmattel @VicDeo