Add a public method to access the Radio state

In order to be sure that the Radio is available to communicate, we now
check to see if the radio is available for commands. We are going to use
this in emergency calling to make sure that the Radio is on (and not
still in the unavailable state) after disabling airplane mode.

Bug: 29039211
Change-Id: I3ee828e165f8e9501de4650c06172ad33c93d51f