Bug 888799: Display message if callee is busy

Busy calls are now signaled by error, instead of call state. This
patch adds a related message to the TelephonyHelper for informing
the user.

The old code used to play a tone for 3 seconds and close the Call
dialog afterwards. We cannot do this any longer because TonePlayer
expects the Call dialog to be around, which has already been closed
when the error gets handled.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Bug 888799: Remove busy state from Dialer

Calls don't have a busy state any more. This patch removes the
related code from the Dialer app. The related test cases have
been removed.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Bug 888799: Added unit test for BusyError

The test verifies that the Dialer app displays the correct error
message when the callee is busy.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>