Fix NPE in CompanionDeviceServiceConnector

Null-check mListener CompanionDeviceServiceConnector.onBindingDied()
before invoking mListener.onBindingDied().

Bug: 225202799
Test: force-stop test companion app that declares multiple &
      CompanionDeviceServices while it "self-reports" companion device &
      as present.
Change-Id: I1ab3cc6d3d8c4f27997012e08b04e28475c9cfe0