servicemanager: Mark vndservicemanager as shutdown critical

vndservicemanager needs to be alive during shutdown to allow vndk
components which may run during shutdown to communicate. The system
ndk servicemanager keeps running throughout shutdown, so this brings
the vndservicemanager on par with that behavior.

Bug: 62951929, 63514363, 62845700
Test: Build
Change-Id: Iba154820ef69c6491a5cb05747b6b905763a825d