Respect existing X-Analytics header, if set

We'd like to manage the X-Analytics header in a single place -- the XAnalytics
extension, introduced in I9137b255a.That way, no extension has a monopoly on
its use. To make migration easy and safe, this patch makes MobileFrontend
respect (rather than clobber) any existing X-Analytics header value.

Change-Id: Id22eaf5dc053650bcd2f9b8dde3ca2460418b902