Fix a NPE in device policy manager service.

It is possible that the device does not have an owner. If there
is no owner we get a NPE when asking the device policy manager
for cross-profile widget providers.

bug:17989189

Change-Id: I5759f2dec160ed8076ab47fdf09134f78c57458d

services/devicepolicy/java/com/android/server/devicepolicy/DevicePolicyManagerService.java

@@ -5320,6 +5320,9 @@ public class DevicePolicyManagerService extends IDevicePolicyManager.Stub {
         @Override
         public List<String> getCrossProfileWidgetProviders(int profileId) {
             synchronized (DevicePolicyManagerService.this) {
+                if (mDeviceOwner == null) {
+                    return Collections.emptyList();
+                }
                 ComponentName ownerComponent = mDeviceOwner.getProfileOwnerComponent(profileId);
                 if (ownerComponent == null) {
                     return Collections.emptyList();