Fix issue that cannot accept third incoming IMS call

When accepting third incoming IMS call with normal swipe action,
background call is disconnected first and then waiting call is switched
with foreground call - foreground call will be held accordingly.
However, background call is still in disconnecting state when calling
switchWaitingOrHoldingAndActive() and then switchingWithWaitingCall
flag is set to FALSE wrongly. This causes third incoming call cannot
be accepted.
Fix switchingWithWaitingCall to TRUE when background call is not alive.

Test: manual - Receive third incoming IMS call
Bug: 62462843

Change-Id: Iae220579e35b7c6e6333d9dcba7061535da88d19