I think that's done by changing the DPI.
Perhaps using something like the Xposed Module 'App Settings' would help.
I'm not sure what the required DPI is to invoke Tablet mode is, though.
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---------- Post added at 06:33 PM ---------- Previous post was at 06:05 PM ----------
Okay, I'm wrong.
After stumbling across a different thread, it seems that Spotify uses a particular method of determining if the device is a tablet.
It's unrelated to DPI, sadly.
A group of people are trying to crack it.
Follow the progress here:
http://xdaforums.com/showthread.php?t=2565626
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