turning off cellular data doesn't do anything when wifi is active and cellular is

unvalidated

The root cause of this bug is:
If you turn on wifi when mobile data is enabled, after WIFI network is ready:

(1) 10s later, ConnectivityService will decrease the APN ref counter and
    deactivate APN mdataEnabled
(2) After 30s, ConnectivityService will finish linger and tear down all mobile
    data connection

However, if you turn off the mobile data in that 30s race window, since APN is
mobile data deactivated already, the turn off mobile data will not tear down
connections anymore. And then if you trun on the wifi, ConnectivityService will
re-enable the mobile data network.

Our fix remove the check of existing data connection and force state change with
the manual UI setting since the enable/disable mobile data should behave like a
switch.

Bug:17282476
Change-Id: If4cc95985c9c7401ef179403274d7f598cbc4a40