Fix NullPointerException in PhantomProcessList due to race condition Bug: 194897294 Bug: 194146206 Test: atest AppChildProcessTest Test: atest CtsAppTestCases:ActivityManagerTest Change-Id: I5d395726d2ff404e8e82d21a7886511a06288c43 (cherry picked from commit 0a05f5475f1f9ea0e61bb69b9c98f4b8ef850b20)