[media] rc: further key name standardization

Use the newly introduced KEY_IMAGES where appropriate, and standardize
on KEY_MEDIA for media center/application launcher button (such as the
Windows logo key on the Windows Media Center Ed. remotes).

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>