init: Cleanly exit subcontext processes upon init's death

This change makes the subcontext processes cleanly exit in the event of
the init's socket being closed. If that was an accident, init will
respawn the process immediately. Otherwise, it will just quietly go
away.

Bug: 80425914
Test: kill -9 $INIT_PID  # Outside of an Android container
Change-Id: I664f11d1b3700ea46857abf24857335fe28e92fa