Settings: Fix crash seen when setting the radio band

When the user presses back before the set radio band
result dialog is shown, crash is seen as the activity
associated with this is already finishing.

Check the activity status before displaying the
set radio band result alert dialog.

Change-Id: I34104773f5cce0be6d0021823b2889aad0fdb945
Author: Jeevaka Badrappan <jeevaka.badrappan@intel.com>
Signed-off-by: Xiaokang Qin <xiaokang.qin@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 39317