Ensure ScheduledExecutor is not shutdown before scheduling timeout cleanup.

The scheduled executor in ConnectionServiceWrapper may be shutdown when calling createConference and createCall. This is infrequently causing a `RejectedExecutionException`. This CL adds a check that mScheduledExecutor is not shutdown before scheduling the cleanup.

Flag: EXEMPT Security High/Critical Severity CVE
Bug: 388588560
Test: manually using the provided apk + atest CallsManagerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:dd63d318cf090ca2d458f772e2799614e6068006)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ef6f10655531a06d2814c3fad25eb7cd1e117581)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:009fdf3e80e7f2c8aaf97d7c87e74f661b0948f3)
Merged-In: I0aaa0f7f57b8dd137403b6ceb7068e7c99652e1f
Change-Id: I0aaa0f7f57b8dd137403b6ceb7068e7c99652e1f