Avoid crashes for no WearableSensingManager in Wear

Wear intentionally removes the backing system service, so the absence of
the service class should not cause a crash/wtf error.

Bug: 340929916
Bug: 367193120
Test: manual, unit test
Flag: android.server.remove_wearable_sensing_service_from_wear
Change-Id: I919948a8d00336563e20d37503c03686433a004d