Check ScheduledExecutor non-nullness before scheduling timeout cleanup

With change ag/30117743 the scheduled executor can be null when calling createConference and createCall. This is infrequently causing a NullPointerException. This CL adds checks that mScheduledExecutor is non-null before scheduling the cleanup.

Change-Id: I5401ff9891829f2dd3e3a1b322bbc55fa400091b
Flag: EXEMPT Security High/Critical Severity CVE
Bug: 379813294
Test: manually using the provided apk + atest CallsManagerTest