Fix NPE in AppExitInfo statsd logging

Bug: 184859606
Bug: 154511130
Test: statsd_testdrive
Test: atest CtsStatsdAtomHostTestCases:AppExitHostTest
Change-Id: I50193d6dd734bfcc8241aa8d4edf487d742926ec

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

@@ -879,7 +879,7 @@ public final class AppExitInfoTracker {
         if (TextUtils.equals(pkgName, processName)) {
             // Omit the process name here to save space
             processName = null;
-        } else if (processName != null && processName.startsWith(pkgName)) {
+        } else if (processName != null && pkgName != null && processName.startsWith(pkgName)) {
             // Strip the prefix to save space
             processName = processName.substring(pkgName.length());
         }