Allow update_engine to kill postinstall process.

This fixes the following denial in O:

 update_engine: type=1400 audit(0.0:2100): avc: denied { sigkill } for scontext=u:r:update_engine:s0 tcontext=u:r:postinstall:s0 tclass=process permissive=0

Bug: 35111618
Test: update_engine_client --cancel during postinstall
Change-Id: I7456a95b5ca6fbdb268a5e16a13e2409758141f5