Fix the issue that dialing *86 during call, the dialing voicemail screen is not shown.

The change fixes the issue that dialing *86 during the call, the dialing voicemail
screen is not shown, instead it shows feature code screen.

For CDMA, when flash is sent to the network, there is no response from the
network to indicate if the feature code is completed or not. This is different
from MMI code in GSM network. So it's confusing to show any UI to indicate MMI
(feature code) for CDMA.

The change is to remove the feature code handling in CDMAPone, and handle the
feature code dialing the same way as the 3 way call dialing. Basically, when
feature code is dialed, the dialing screen will be shown.