ConnectivityService: Don't reconnect to mobile data if it is disabled

Disconnecting wimax triggered a reconnect to mobile data without
checking if it was enabled at all, and this was causing mobile
data to be used even against the user's configuration. For some
reason, this is written to affect wifi disconnects, too. Why?

In any case, add the mobileDataEnabled() check.

Change-Id: Ifdd8f0273e359baad5ef58ac14b59cfa399b5d84