PackageManager#queryIntentServices breaks its contract.

PackageManager#queryIntentServices javadoc contract states that this
method (and the like) never returns null, rather an empty list if
no result is found. However, there is a path in the PackageManagerService
that returns null, thus breaking the contract. Handling the null list
explicitly.

bug:10930560

Change-Id: I708c51b8b7075e529145c8b0bf159efd6b697532
diff --git a/services/java/com/android/server/print/PrintManagerService.java b/services/java/com/android/server/print/PrintManagerService.java
index 5f8708a..9810bf0 100644
--- a/services/java/com/android/server/print/PrintManagerService.java
+++ b/services/java/com/android/server/print/PrintManagerService.java
@@ -413,6 +413,10 @@
                         .queryIntentServicesAsUser(intent, PackageManager.GET_SERVICES,
                                 getChangingUserId());
 
+                if (installedServices == null) {
+                    return;
+                }
+
                 final int installedServiceCount = installedServices.size();
                 for (int i = 0; i < installedServiceCount; i++) {
                     ServiceInfo serviceInfo = installedServices.get(i).serviceInfo;