Fix MT Call screen stuck issue.

    The issue this change fixes can be reproduced by following steps:
    1. Originate and connect a call to remote party #1 that is not in Contacts.
    2. Receive a call from the remote party #2 that is not in Contacts, the
       call waiting tone is sounded and the call waiting screen is displayed.
    3. Press menu key and ignore the call, and the call waiting screen is
       dismissed and the remote party #2 is forwarded to voice mail.
    When we ignore the incoming call, Phone gets stuck with incoming call
    screen.

    The fix is:
    1. Update phone state promotly based on the call waiting state.
    2. Remove new call handling in handlePollCalls() function because in CDMA,
       the call drop should be determnited by the number of calls in the call
       list.