Change preference name for enabling MediaViewer

Should simply say 'Enable Media Viewer' instead
of puzzling our users with 'Enable new media
viewing experience'.

Requested on Commons, Mingle card 622.

Change-Id: Ibe6c11a9fe4b3fd64de65ae50055e3fc377c3ae9
(cherry picked from commit 826ac9b69d7d8f8788f4950cabaf6736485c6d05)