Fix issue where incorrect call state disables "Home" button.

There is a potential, due to multithreading, for a call to enter a
ringing state, become disconnected, and then finally be added to Telecom
where it will again be set to ringing.  The PhoneWindowManager code
uses the TelecomManager API to check if there is a ringing call and to
determine if the home button should be disabled.  The aforementioned
circumstances result in the PhoneStateBroadcaster not properly tracking
the current call state.  It happens that the Telecom API isRinging method
simply checks the current state in the PhoneStateBroadcaster.  In the
scenario for this but, the phone state is stuck in ringing, and as a
result the home button remains disabled.

This CL fixes this on 2 fronts:
1. Correct CallsManager onSuccessfulIncomingCall so that it will not
automatically set the state of a new incoming call to ringing if it has
already been disconnected (this will prevent PhoneStateBroadcaster from
getting the wrong state).
2. Change the Teleco isRinging API to check CallsManager#hasRingingCall(),
which will look at the list of currently tracked calls to determine whether
there is a ringing call (regardless if the PhoneStateBroadcaster has a
potentially inconsistent state).

Bug: 25146970
Change-Id: I6273c72bac5fc7d91e16a8ff8aa12a7081089824