Don't needlessly disable data.

If data is set to SIM2 and one comes into range of a Wifi network,
currently setDataAllowed(false) is called for SIM2. Some radios don't
seem to like that. As that doesn't happen to SIM1 in the same scenario,
make the behaviour consistent for both and only call disable data when
toggling data between SIMs.

Change-Id: I0d08ccb7d3f33d6447b187c100708399d8e7ec87