varnish: adjust first_byte_timeout for mobile too

Bring mobile varnish TTFB timeouts to parity with their desktop
equivalents, as previously fixed with
Ic14902d399712a1eb0d2d98ce5f505e6740ef68b.

Bug: 57249
Change-Id: I6896b6863668641061ffa1174eae7f36f3d71544