init: fix restarting of subcontext

When the subcontext code was redone to allow only one subcontext
(vendor_init), the code for restarting it and for terminating it
during shutdown was not updated, resulting in it not working.

Bug: 155203339
Test: kill subcontext init and notice it restart
Test: subcontext init stops during shutdown
Change-Id: Ib77f59d1e7be0ffcfd3f31c8450dc022c20bb322