Allow update_engine to suspend/resume postinstall.

update_engine launches the postinstall process and can suspend and
resume it by sending SIGSTOP and SIGCONT. This fixes the following
denials:

update_engine: type=1400 audit(0.0:88): avc: denied { sigstop } for scontext=u:r:update_engine:s0 tcontext=u:r:postinstall:s0 tclass=process permissive=1
update_engine: type=1400 audit(0.0:89): avc: denied { signal } for scontext=u:r:update_engine:s0 tcontext=u:r:postinstall:s0 tclass=process permissive=1

Bug: 28959137
TEST=`update_engine_client --suspend ; update_engine_client --resume` while the device is running postinstall.

Change-Id: I9890ad0ff7fe04bae1a54fa07c61aafca8de8e66