init: Cleanly shut down subcontext processes

This change adds an explicit cleanup for the subcontext processes and
avoids them from respawning, which causes a bunch of LOG(FATAL)s when
the system is going down.

Bug: 80425914
Test: kill -TERM $INIT_PID, no crashes for subcontext inits

Change-Id: I135191d959c1dd921b102af316b24d2bc161d6c9