Settings: Fix settings force close in monkey test.

Unable to start activity ComponentInfo{../...settings.SubSettings}: java.lang.IllegalArgumentException: Cannot add a null child view to a ViewGroup.

Forece close caused by mUnmount is null. Add check to avoid.

Change-Id: I4e4e2568f488b79e2090de27ca05858467927474
CRs-Fixed: 1084021