Use killProcessGroup when killing app zygote.

To make sure we kill all untracked children, too.

Bug: 156741968
Bug: 157598956
Test: manual inspection, PoC no longer works.
Change-Id: I5d8efeb05ddec08a7fc7c00eabca6590c4cfdd8c

core/java/android/os/AppZygote.java

@@ -92,10 +92,9 @@ public class AppZygote {
     @GuardedBy("mLock")
     private void stopZygoteLocked() {
         if (mZygote != null) {
-            // Close the connection and kill the zygote process. This will not cause
-            // child processes to be killed by itself.
             mZygote.close();
-            Process.killProcess(mZygote.getPid());
+            // use killProcessGroup() here, so we kill all untracked children as well.
+            Process.killProcessGroup(mZygoteUid, mZygote.getPid());
             mZygote = null;
         }
     }