Can't be too sure - being the headunits fitted in AU tend to be different to those elsewhere - however P4-140 (approx) - Audio System - onward of your user manual may be useful.
Working on the basis you are using spotify on your mobile device then streaming audio via BT (rather than the head-unit supporting it and requiring mobile for data only). So more specifically P4-175 (approx) Bluetooth Wireless Technology AUDIO probably applies - however it doesn't actually cover individual use too much. But has a note saying along the lines... some phones won't support some functions. Which implies you should be able to use the last/next - but it could depend on your phone... I have noticed though that it can be a bit slow in doing so with my head-unit - button needs to be a definite press and then a wait of a second or two before track change.
On the basis that the last/next are simply indicated key-presses to do an action as part of the BT audio profile, so once on the mobile device shouldn't be treated any different to other key presses - I can't see why it wouldn't work with any media played on the mobile device (assuming the device itself accepted/knew what to do with them).
