DcTrackerBase: Dont provision APN if default is disconnecting

Change-Id: I99344d89235b24689cde14335bf593099169ef0d
DcTracker: Don't leave CMD_IS_PROVISIONING_APN dangling

Change I99344d89235b24689cde14335bf593099169ef0d exited the message
handler without a reply to the caller, leaving the singlethread queue
waiting for a reply and blocking other async tasks.

Change-Id: If12361c0037e9ca0662d720acfe55215924b67fd