Fix NullPointerException in PhantomProcessList due to race condition

Bug: 194897294
Bug: 194146206
Test: atest AppChildProcessTest
Test: atest CtsAppTestCases:ActivityManagerTest
Change-Id: I5d395726d2ff404e8e82d21a7886511a06288c43

services/core/java/com/android/server/am/PhantomProcessList.java

@@ -365,6 +365,9 @@ public final class PhantomProcessList {
     private int onPhantomProcessFdEvent(FileDescriptor fd, int events) {
         synchronized (mLock) {
             final PhantomProcessRecord proc = mPhantomProcessesPidFds.get(fd.getInt$());
+            if (proc == null) {
+                return 0;
+            }
             if ((events & EVENT_INPUT) != 0) {
                 proc.onProcDied(true);
             } else {