Bug 859736 - Speaker is disabled when rejecting an incoming call during an established call (r=etiennesegonzac)

To avoid weird behaviors regarding the speaker and mute functionality, we reset them when handling the first incoming call. For the proposed patch we have considered the case when a busy tone is played in which case, although the call is disconnected, the call screen is shown. If while the busy tone is played, a new call reaches the phone, it is properly reset to not muted and the speaker off.