Bug 859736 - Speaker is disabled when rejecting an incoming call during an established call

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.