Bug 875719 - Simplify and correct the mechanism of handling call state change event. r=gyeh, a=tef+

1. Remove variable mCurrentCallIndex and replace with using function FindFirstCall(aState)
2. Remove unused variable index and callArrayLength
Bug 864588 - [B2G] [Leo] [Internet Sharing] USB tethering turns off almost immediately after being turned on. r=vchang, a=leo+