Kill the child process when the parent process dies.

When we have a process hierarchy, such as artd->odrefresh->dex2oat, we
want to be able to kill a subtree. For example, if we kill odrefresh, we
want dex2oat to be killed as well; if we kill artd, we want both
odrefresh and dex2oat to be killed as well. This can be achieved by
PR_SET_PDEATHSIG.

Bug: 311377497
Test: Kill odrefresh and see dex2oat killed.
Change-Id: I28cc271f5c37b9ad50bee179eb9b211cb3545a2f