Prompt and dialog fixes, preference addition

Added developer preference to toggle allowing the background
service staying alive outside of the AppRTC activity (default: on).
Changed authentication prompt and progress dialog to no longer be
cancellable by clicking outside the box. To cancel the auth prompt
you must click the "Cancel" button, and to cancel the progress
dialog you must click the back button.
Fixed bug where double-tapping an item in the connection list would
result in a nasty crash.
Fixed bug where lack of a session token would trigger an auth
prompt to re-open an already open connection.