Zygote: Unblock SIGCHLD in the parent after fork.

Follow up to change e9a525829a. Allows the zygote to
receive SIGCHLD again and prevents the zygote from getting into a
zombie state if it's killed.

Change-Id: I81fde39a2dd32eaf7d8b397672d3165a4eb945e1
Contributed-By: rhed_jao <rhed_jao@htc.com>
Bug: 32693692
Test: manual

core/jni/com_android_internal_os_Zygote.cpp

@@ -628,6 +628,12 @@ static pid_t ForkAndSpecializeCommon(JNIEnv* env, uid_t uid, gid_t gid, jintArra
     }
   } else if (pid > 0) {
     // the parent process
+
+    // We blocked SIGCHLD prior to a fork, we unblock it here.
+    if (sigprocmask(SIG_UNBLOCK, &sigchld, nullptr) == -1) {
+      ALOGE("sigprocmask(SIG_SETMASK, { SIGCHLD }) failed: %s", strerror(errno));
+      RuntimeAbort(env, __LINE__, "Call to sigprocmask(SIG_UNBLOCK, { SIGCHLD }) failed.");
+    }
   }
   return pid;
 }