fix NPE in CSW#oCC when the ServiceInterface is cleared out

A crash cluster revealed a race condition that causes an NPE on CF
devices. Although this is not repro on local devices, a fix should
be merged.

Fixes: 364811868
Test: build + ran the failing test
Flag: com.android.server.telecom.flags.csw_service_interface_is_null
Change-Id: I05f5a767c47e43f163335dc87a78ea883a55f277