Send updateHeadsetPhoneState when child calls change

In BluetoothPhoneService, don't debounce the notification of the state
change if the number of child calls change. It might take a while before
children are added to a conference, so we should inform the headset that
something changed so that it'll actually pick up on the child calls.

Change-Id: Ia7d458556cf8ec96293a677076c58dcaab0d7387
Test: manual
Fixes: 64041266