Whitelist API mobileview for robots.txt

Per request from Google.

Will allow Googlebot etc to run scraping through the Android app
and through the mobile web site with active JS as we add more
dynamic section loading to it.

Note that while these API requests probably do not get fully HTTP-cached
they do aggressively use the parser cache, and should be directed to
API servers to limit load.

Expected activity from Googlebot is maximum 50 hits/second, which will
be a modest increase over existing traffic of approx 150 hits/second
from apps and mobile web.

If there are performance issues, please feel free to revert this change
and contact deeplinking-tt-support@google.com to let them know there
were problems.

They also recommend returning 503 errors as the best kill switch!

Change-Id: I753eebd965bf7f6b63ad734fa728379b3701ad73