Prompt user to add data on mobile portals

On mobile networks it is more likely that the user needs to top up or
get a mobile data subscription, rather than "sign in" as the previous
messaging was suggesting.
Following feedback from interaction designers / UX writers, change the
strings to be more appropriate for the most common use-case.

Bug: 153673376
Test: atest FrameworksNetTests
      manual test on mobile network force-detecting a portal.
Change-Id: I5b4d4a17fd507d8cf3c9863cf23b5fedb258852e