Fix Amazon Silk detection

Per http://docs.aws.amazon.com/silk/latest/developerguide/user-agent.html
Silk (formerly known as Kindle), doesn't send the usual "Mobile" word in user-agent
when in "desktop" mode, however we want to redirect it even then.

Change-Id: Ib423261fd5a8ba1b3ff74257f0fcd78a9e7dc35a